Need to add a custom item, but don't want to sacrifice a vanilla item by reskinning it? Blank Items has you covered. The mod adds 50 new items with no names, textures, or properties - the perfect opportunity for you to add your own. It's quick and easy to make a resource pack that names and textures them, and they can then be added to crafting recipes or otherwise integrated into the game with data packs.
The items' IDs are "blankitems:item_1" through "blankitems:item_50".
A template resource pack is provided for 1.16 - I use it to make some relatively endgame crafting recipes.