Oh I didn't know about the Map inventory plugin! I looked at Kagedesu's video, and I think that plugin has already basically accomplished what I'm trying to do (visual chests, storage, grid inventory, etc.)
It would probably take not much work for him to just add support for custom item shapes to his plugin.
The work I did here is mostly just understanding how to work with grids cause I needed it for "large monsters" in my grid battle system, and maybe a game of Tetris, so if his plugin is good enough I'd probably just go with that one.