Minecraft Version

1.20.1

Describe the Bug

TradeBoxBlockEntity exposes unneeded ITEM_HANDLER (inherited from BaseContainerBlockEntity) capability effectively allowing one to place hopper (or other means of getting items out of the container) to bottom to get everything that's in the Box.

Hmm... Seems like not your issue but Forge's. Hopper doesn't seem to care about capabilities and just grabs items found in BE's Container...

Same problem on 1.19.2
Easiest solution i can think of is to restrict which inventory slots can be accessed from each side, like the vanilla furnace.