Scarfing the bottom is the same as the sides. I learned controlling all the pieces is the trick. The two pieces are glued in a sandwich. I did a dry run, then stapled the glass tape and plastic strips. Also after butting the back pieces in the actual gluing, I stapled them out at the edges to keep them from slipping around.
Concrete blocks work fine for weights.
I suspect two curved boards faced with Formica and release agent could be nice if I did this a lot. Some 40 grit sand paper attached to the edge of the level makes a good tool for preparing the sides for the bottom. Body shop supply stores sell these for fairing car bodies. They come with peal and stick adhesive and work great. The bottom goes on.