Ignore the ugly chalk marks. My chalk line came with permanent black chalk which got over everything. A builder friend recommended using baby powder instead. So far I’ve made do with a 4′ straight edge and a pencil line instead. This pic shows my original idea of leaving an overhang so that the rigid foam board going around the outside of the frame would have something to sit on. I got worried that this edge would get crushed when we flipped it over and might cause problems when we tried to fit the frame in the trailer, so I ended up cutting it off. The rigid foam board rests on the cross ribs and is bolted in so it’s not going anywhere and because it’s closed cell, it won’t be affected by water.