4x8sheet $69.30usd and a little more durable final skin
8mm Twin-Wall Polycarbonate Sheet - 47.25"W x 96"L
Support required depends on pressure balance and forces involved
The one being a picture called a picture frame I see as a bird cage support structure you would either bend the colorplast over the framing or cut panels to fit between the frame sections like gussets.
Pressure balance as mentioned is critical to dealing with off angle wind or side wind. Couple of days ago driving South through Idaho in a ground blizzard even pressure balance was very important the front tires would actually slip a little as they hit the ice first giving me time to respond with the drive tires