I recently covered a four star 120 with monokote. Is this structure open or sheeted? If it's open and sturdy like a wing, follow the directions on the product and tack the covering to one end and pull it as tight as possible as you tack it to the structure in various places. Then shrink it taught with the heat gun. If you are doing a structure like a turtle deck, same thing but count on spending a TON of time and heat to get it perfect. Keep heating it till you are sure it'll burst into flames. Right before it does, it'll be perfect.....

If you are covering a sheeted structure follow your gun with a clean oven mitt for the most beautifull glossy surface.

After rereading your post, Ill say this, MORE HEAT. It is frustrating when one open bay gets tight and the one next to it is disturbed. See if you can work it in one direction to the end of the structure. Oh and MORE HEAT........It will eventually pull tight........