There is an idea for a small square tuck box. If the idea gets enough votes, they will look into what it will take to offer it. It mostly depends on if there will be enough demand I think.
Currently, we don't offer custom shapes and sizes on a one-off basis. As Zovu stated, we accept suggestions on our Ideas page, which can then be voted on by members of the community. We try to implement the most popular ideas whenever we can.