Watching this youtube video on boat shrink wrapping helped me figure it out.
The staples act as the perimeter band to keep it down, but the rest is basically the same concept.
I tied the two sides together with a board across the bottom inside because the plastic tends to compress the whole structure as it shrinks and I didn't want it to distort the frame. I don't know how much force it can put on the structure, but I wanted to be safe.