By Eris it works on MP.

src/main/java/net/brokenmoon/afloydironchest/blocks/SteelChest.java

@@ -57,10 +57,10 @@ public class SteelChest extends BlockContainer{
     public static void displayGui(EntityPlayer player, IInventory inventory){
         if(player instanceof EntityPlayerMP) {
             //Multiplayer
-            ((IEntityPlayerMP)player).displayGUIIronChest(inventory);
+            ((IEntityPlayerMP)player).displayGUIDiamondChest(inventory);
             return;
         }
         //Singleplayer
-        ((IEntityPlayerSP)player).displayGUIIronChest(inventory);
+        ((IEntityPlayerSP)player).displayGUIDiamondChest(inventory);
     }
 }

src/main/java/net/brokenmoon/afloydironchest/gui/ContainerWideChest.java

@@ -4,28 +4,26 @@ import net.minecraft.src.*;
 
 public class ContainerWideChest extends Container {
     private IInventory inventory;
-    private int numberOfRowsLower = 4;
     private int numberOfRowsUpper;
 
     public ContainerWideChest(IInventory lowerInventory, IInventory upperInventory) {
         this.inventory = upperInventory;
         this.numberOfRowsUpper = upperInventory.getSizeInventory() / 12;
-        int i = (this.numberOfRowsUpper - 4) * 18;
         //Upper
         for (int j = 0; j < this.numberOfRowsUpper; ++j) {
             for (int i1 = 0; i1 < 12; ++i1) {
-                this.addSlot(new Slot(upperInventory, i1 + j * 12, -18 - 6 + i1 * 18, -36 + j * 18));
+                this.addSlot(new Slot(upperInventory, i1 + j * 12, 8 + i1 * 18, 8 + j * 18));
             }
         }
         //Lower
         for (int k = 0; k < 3; ++k) {
             for (int j1 = 0; j1 < 9; ++j1) {
-                this.addSlot(new Slot(lowerInventory, j1 + k * 9 + 9, 3 + j1 * 18, 49 + k * 18 + i));
+                this.addSlot(new Slot(lowerInventory, j1 + k * 9 + 9, 8 + 18 + 9 + j1 * 18, 174 + k * 18));
             }
         }
         //Taskbar
         for (int l = 0; l < 9; ++l) {
-            this.addSlot(new Slot(lowerInventory, l, 3 + l * 18, 49 + 18 * 3 + 4 + i));
+            this.addSlot(new Slot(lowerInventory, l, 8 + 18 + 9 + l * 18, 232));
         }
     }
 

src/main/java/net/brokenmoon/afloydironchest/gui/GuiDiamondChest.java

@@ -9,24 +9,14 @@ public class GuiDiamondChest extends GuiContainer {
     private IInventory lowerChestInventory;
     private int inventoryRows;
 
-    public int xSize = 240;
-    public int ySize = 222;
-
     public GuiDiamondChest(IInventory iinventory, IInventory iinventory1) {
         super(new ContainerWideChest(iinventory, iinventory1));
         this.upperChestInventory = iinventory;
         this.lowerChestInventory = iinventory1;
         this.field_948_f = false;
-        int c = 222;
-        int i = c - 108;
         this.inventoryRows = iinventory1.getSizeInventory() / 12;
-        this.ySize = i + this.inventoryRows * 18;
-    }
-
-    @Override
-    protected void drawGuiContainerForegroundLayer() {
-        this.fontRenderer.drawString(this.lowerChestInventory.getInvName(), -24, -47, 0x404040);
-        this.fontRenderer.drawString(this.upperChestInventory.getInvName(), 3,  128, 0x404040);
+        this.ySize = 256;
+        this.xSize = 230;
     }
 
     @Override
@@ -36,15 +26,6 @@ public class GuiDiamondChest extends GuiContainer {
         this.mc.renderEngine.bindTexture(i);
         int x = (this.width - this.xSize) / 2;
         int y = (this.height - this.ySize) / 2;
-        int h1 = Math.min(this.inventoryRows, 6) * 18 + 17;
-        this.drawTexturedModalRect(x, y, 0, 0, this.xSize, h1);
-        int rows = this.inventoryRows;
-        while (rows > 6) {
-            int h2 = Math.min(rows, 6) * 18;
-            this.drawTexturedModalRect(x, y + h1, 0, 17, this.xSize, h2);
-            rows -= 6;
-            h1 += h2;
-        }
-        this.drawTexturedModalRect(x, y + this.inventoryRows * 18 + 17, 0, 126, this.xSize, 96);
+        this.drawTexturedModalRect(x, y, 0, 0, this.xSize, this.ySize);
     }
 }

src/main/java/net/brokenmoon/afloydironchest/mixin/MixinEntityPlayerMP.java

@@ -29,7 +29,7 @@ public class MixinEntityPlayerMP implements IEntityPlayerMP {
     public void displayGUIDiamondChest(IInventory iinventory) {
         this.getNextWindowId();
         NetServerHandler.logger.info(((EntityPlayerMP)(Object)this).username + " interacted with chest at (" + ((EntityPlayerMP)(Object)this).posX + ", " + ((EntityPlayerMP)(Object)this).posY + ", " + ((EntityPlayerMP)(Object)this).posZ + ")");
-        this.playerNetServerHandler.sendPacket(new Packet100OpenWindow(this.currentWindowId, 0, iinventory.getInvName(), iinventory.getSizeInventory()));
+        this.playerNetServerHandler.sendPacket(new Packet100OpenWindow(this.currentWindowId, 7, iinventory.getInvName(), iinventory.getSizeInventory()));
         ((EntityPlayerMP)(Object)this).craftingInventory = new ContainerWideChest(((EntityPlayerMP)(Object)this).inventory, iinventory);
         ((EntityPlayerMP)(Object)this).craftingInventory.windowId = this.currentWindowId;
         ((EntityPlayerMP)(Object)this).craftingInventory.onContainerInit(((EntityPlayerMP)(Object)this));

src/main/java/net/brokenmoon/afloydironchest/mixin/MixinNetClientHandler.java

@@ -0,0 +1,29 @@
+package net.brokenmoon.afloydironchest.mixin;
+
+import net.brokenmoon.afloydironchest.MixinInterfaces.IEntityPlayerSP;
+import net.brokenmoon.afloydironchest.tileEntities.TileEntityDiamondChest;
+import net.minecraft.client.Minecraft;
+import net.minecraft.src.NetClientHandler;
+import net.minecraft.src.Packet100OpenWindow;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(value = NetClientHandler.class, remap = false)
+public class MixinNetClientHandler {
+
+    @Shadow
+    private Minecraft mc;
+
+    @Inject(method = "handleOpenWindow", at = @At("TAIL"))
+    public void injectMethod(Packet100OpenWindow packet100openwindow, CallbackInfo info) {
+        if (packet100openwindow.inventoryType == 7) {
+            TileEntityDiamondChest dchest = new TileEntityDiamondChest();
+            ((IEntityPlayerSP)this.mc.thePlayer).displayGUIDiamondChest(dchest);
+            this.mc.thePlayer.craftingInventory.windowId = packet100openwindow.windowId;
+        }
+    }
+
+}

src/main/resources/afloydironchest.mixins.json

@@ -6,7 +6,8 @@
     "mixins": [
         "MixinEntityPlayerMP",
         "MixinEntityPlayerSP",
-        "MixinTileEntity"
+        "MixinTileEntity",
+        "MixinNetClientHandler"
     ],
     "client": [