It's been about 25 years since I built my frame using handrail fittings. At that time, I was able to find curved sections of aluminum pipe that matched the shape of my raft. I used these to span between the inner top rail and the outer rail of the frame floor. That allowed maximum interior space and maximum contact between the frame and the raft.
Bridging between the top rails was not necessary. The oarlocks provide one connection, and the two crossbars on the front and rear provide the rest.
The aluminum rails are 1" Schedule 40 aluminum pipe. The fittings are Hollander Speed-Rail.