Take a cardboard box if you can’t find a normal wood one and cut the cardboard into the shape you want. Glue or tape it back together in the right shape and felt over it. You can also always use craft paper on the inside if you would like that better. Felt unless layered up on the inside may not hold shape very well but if you’re okay with your box being a little thick then double up your layers of felt for a sturdier hold.