Ventured to the hardware to buy some 9mm ply - went crazy and got marine ply for the seat bases. Cut to width, first fit of the base. So far, this was going easily. Back in more familiar territory with timber rather than steel. Marked the radius, cut, fitted. Drilled holes, countersunk, self drilling screws to fix I'm paranoid about things moving after I have measured / marked out. Things going far too well, on to the back. Clamps required. Radius marked and cut, fitted and fixed. Things going far too well at this point, I should have been suspicious, and issued another moderate coarse language warning. The bed would fold out less flat than before. Not that obvious there, but the timber going 1mm past the steel in the hinge area meant it wouldn't go flat. Aware of my ability, or lack of it to repeat fit the ply, I operated on it in situe with jigsaw, rasp and sander. Got it folding back to the original level of flatness. And pleased with myself again. Looks like a seat and bed to me!