you do need to measure correctly at the bottom so the ridges interlock properly on the last piece.
That was my concern, but I don't think measuring will work in real life.
Might be better to do it backwards: run a panel right down the middle of the top, and then slip the panels on either side
under the edge of the panel above it before screwing down the edges
Somebody offered to us the frame for one of those portable soft cover carports (the cover of course long since gone). We were thinking of making a greenhouse out of it, by running some translucent panels (lexan?) lengthwise.
Also, another neighbor gave us a whole pile of timbers from the garage that they tore out to build a new metal one. As soon as the snow melts and the mud dries we're going to haul them over here. What I would like to do is build a detached root cellar with them