About half the responses here seem to be suggesting forcing a square peg into a round hole. Which is a terrible mindset to approach your projects with, in my opinion.
If your design demands a box collider instead of a capsule, you write your own character controller instead of trying to jury-rig ones that have been written for you. Plain and simple.
Don’t be lazy. Do the leg work and learn some valuable skills in the process. Otherwise you’ll be trying to jury-rig your entire game, doing as little work yourself as possible, and learning next to nothing.