The two methods i have used, although not for Ferret boxes are:-
1. Use two very thin ply sheets, that bend easily, then laminate them together by gluing. It retains the bend very easily, because they are different radii and cant move once the glue is set.#
2. On the inside radius of the bend or thicker plywood, make regular cuts with a depth controlled circular saw, then apply glue to the cuts and bend.
If you have cut an arc in plywood making the top and bottom, it should be easy to hold in place with a few panel pins, whilst the glue sets. unfortunately, i cant tell you the frequen