Custom Container Types

[Wonderlarr]

In my mod, I have a few items that have a modular upgrade system using an inventory slot, all of these, except for one, are Hat's. When equipped, it looks like Walter's slingshot ammo slot, like this

My problem here, is actually also in the image, both the Purple Hat and the Blue Umbrella thing should have that slot above them, but it seems since they both share a container type, only one can be open at a time.

When I tried to specify my own container type, I no longer had some of the important properties that hand_inv, which is the container type I'm using right now, has. Specifically how it's integrated into the hotbar, and moves with it if you have a backpack on with integrated backpack mode enabled, and it also has some slight size modification to the slot and other things. How would I go about making my own slot similar to hand_inv?