Dave, Steve Cox has it pretty well nailed down in his post. I myself thought that the PolyMax was a HDPE product, (High Density PolyEthelene) from the information I get from Farm Tek. I have tried heating the stuff with a soldering iron but unless you go just about all the way through both pieces it will not even begin to stick, even then the joint was very weak.
The shipping weight for the 5'x10' piece was about 15 pounds, so it is relatively light stuff. I agree with Steve insofar as the vacuum effect using Henrys. I may try a bit of that myself. However on the seams covered by aluminum and screwed down every 6 or 8 inches with a good bead of the Marine GOOP stuck in there. I beleive that it will make a very good seal. These pieces where joined will have virtualy no movement to disturb the relatively flexible seal provided by the GOOP. The extra screws and bonded washers up the center and on each spar are just for good measure, so it will not bubble up like an old convertable top.
Guess I gotta get me an avitar of some sort, huh.....