There is still the problem of custom backpacks only letting you use only the left-side of the pack though, so if anyone has figured out how to work around that problem, please post it here.
After looking into this more
here and having
@Lokai look into it as well, we're leaning toward the client being what tells how large of square space itemids get.
Never fear! There is an alternative "work-around". In order to get custom
full-size waorking backpacks, all that you have to do is use an itemid that is allowed the full space. There are almost 400 container items in the newest client (), but don't worry, I already went through each one and checked their backpack "square footage" for you. I have put my findings into a spreadsheet so that you can easily see which itemids I have checked, versus which ones allow the full backpack to be used.
In short, not including the default 2 backpack (mirror images) and not including ship hulls, there are 5 additional options to take advantage of the full backpack size and 3 additional options that almost take advantage of the full backpack's size.
5 full size backpack ids:
0x09B0 belt pouch
0x0E1C backgammon board
0x0E79 pouch
0x0FA6 game board
0x0FAD backgammon game
3 almost fullsize backpack ids:
0x261B Plague Beast Lord
0x4D0C Kings Collection Box
0x4D0D Kings Collection Box
If you are wondering what I mean by not being able to use the full square footage of the backpack, I compared screenshots of the how much space you get in each by placing a gold ring in each corner of the square (see below):
Plus having Lokai helping out!