OK! Lets talk about H.E.L.M. More so, lets talk Mining! Mining has always been the backbone of Smithing and Goldsmithing. So when talking about making Items stackable, how about ore! Now, ores themselves, should not be stackable, but I propose this:

Create an Item for each ore; "Copper Ore Sack" "Zinc Ore Sack" and "Iron Ore Sack" just to name a few. For example, it takes 12 zinc ore, 36 copper ore, and 12 fire crystals to make 1 stack of Bronze Ingots. That is 49 slots on your inventory just to make one stack of ingots. And that is saying you toss everything that does not make Bronze.

The items could resemble the "Water Tank" - an item that has "X" charges to dispense "X" items. But on top of that, make it where you can "Trade" items into it (Using the menu that the crystals use during a synthesis, for example). As you mine ore, you place it into the sack. And when the sack is full, its full. But you don't use your entire innovatory to do so! This item may be sold on the AH. That way when you go to buy ore, your not just buying singles. Not to mention, if you can make Bronze Ingots, but not high enough to make Iron, but you have a full "Iron Ore Sack" from mining in Zeruhn Mines, you could sell that for gil to make money to further your crafting or just sell to make gil, period.

This item could be bought from an NPC, crafted, or received from the Guilds somehow.