Here's a material that is essentially coroplast with an aluminum skin on both sides:
AlumaCorr. I haven't found a local supplier.
I've started a removable, redwood framed and shrinkwrapped, test boattail. I'm hoping my camera shows up before I get too far along. I intend to do the final version in
Polymetal. It's $148 for a 4x8' piece, $104 if I have a shop order it for me.
It sounds like you have six attachment points. I would think with the reduced base pressure there wouldn't be too much suction trying to pull it off. Vibration and tail-wagging must be the bigger problems. I plan on two roof rack style clamps on the driprail and pegs into sockets already in the bumper brackets. With some straps to hold it down.