Cardboard Box
Cardboard Box · block · Catgirl & Catboy Mod
Place the box and right-click it to climb in. You sit inside with your head sticking out and can't move. Sneak to climb out. One box holds one player.

While you are inside, mobs can't pick you as a target. Mobs within 32 blocks that were already chasing you drop the chase the moment you hide. Climb out and they can see you again.
The first time you sit in a box you get the advancement Box Enjoyer.
A Kitten walks to a box within 8 blocks, hops on top and sits there.
Crafting
Paper × 5
Crafting table.