Just too cool for words
Here is an old trick from an old shade tree bodywork guy (actually an old race car driver). To eliminate dips from bondo work, not much different from your problem, I used a full 8 1/2 x 11 sheet of sandpaper stapled to a dressed 1 x 4 x 11" block. Cut the block to the long dimension of the sandpaper, 11". Wrap the sandpaper tightly around the block and staple it along the edge of the block. A standard sheet of sandpaper just exactly fits without any trimming. The longer length of the block is much more effective eliminating dips than the shorter sanding blocks.
Also, once you have the fiberglas covering into place, you will be able to add small quantities of bondo to smooth any remaining dips, so it isn't ultra critical.
I wonder a bit about that sharp edge on the bottom outsides. As I think I remember from Darin's Metro tail, there is flow "down" the lower tail to repressurize the diffusion area underneath. As I'm sure you have noticed, there is a bit of a problem extending the stock Insight line since it is quite sharp on the fender skirts and bumper. I wondered just what to do with this problelm myself, but it would seem that continuing the sharp line through the length of the boattail would not be correct. Clearly you can round this area considerably by installing extra foam inside the corner. What do you think?
I see from the photos that you had a very productive weekend
