That sounds like an interesting approach. Obviously it works if you are doing it. You are certainly right about it being a good idea to nock the square corners off of the edges of the plywood sheet. It does indeed help with getting the sheets to fit into the channel. That is intersting that your frame does not have any lower cross-members. I guess the Argosy product line did a few things differently.
Thank for sharing your approach. Do you have any photos of the work underway?
Only he who attempts the ridiculous can achieve the impossble.