Sorry for the lag, folks.
After some sketching and noodling, I have a slightly different idea for attaching the fabric for the top and sides at the top rim.
Here's a *rough* sketch....



This is a profile as if you're looking right at the lid.
The top fabric will overhanf the sides by about 3", and snap to the sides there. The side fabric will also use snaps (yellow in pic), attaching just below this, then stretching down to the bottom rail at the truck bedsides.
Since this leaves a gap between them, this will be covered with an additional strip of fabric (abyne 1.5,2" wide) sewn to the top peice and overhanging the side... idea being that rainwater will flow over it and down teh side, and not inside.

Thoughts? The point of doing it this way is to make it easy to removet he side panels and leave the top on. My only concern with this setup is that the bit hanging down past the snaps might flap around when driving on the highway etc.