Rebuilding convertible rear seat bottom frame

I would have to assume that the rear seat regardless of convert or not would use the same basic steel rail components.
My point is if you have another seat bottom, for instance from a hard top, that sections of the frame would have the same formed shape and except for curved sections you should be able to cut and splice as needed.

Alternatly you could cut out the bad and form one patch at a time and weld it in.