You can do this, but there are limits. I mad an impromptu back deck for fishing this spring from 3/4" ply and varnish. Hung it with straps through routered slots From the back of my frame and a d-ring on the rear tube.
I cut the ply to fit the shape of the tubes, so the unanticipated part was that if/when you stepped into or out of the boat, the whole floor would shift on you and could potentially chuck you out of the boat.
I resolved the issue by extending the casting brace through an offset-T to a pivoting hollaender cup that's bolted to the floor. The tube gets pinned or screwed into both the cup and the T. This provided enough force to keep the floor from flipping on the fisherman, while its primarily supported by the straps on the outside corners.