I would guess the easy way would be to use a backpack item with a sprite replacer to be a keyring. If memory serves, you could find backpacks and use them as a sort of inventory sorter (at the nice price of like ten pounds a piece or something) I'm unsure if it was Fallout Fixt that added them or something, but I remember doing that for misc ammo and storyline tapes/drives/keys and the like.