Hi Adam,
I did the same thing last year to and from BM (starting from West Marin).
I'm pretty sure they were 1/2" thick. So I had two 4x8 plywood boards, one on top and one on bottom.
I did not wrap the plywood boards in order to give the tarp more protection from below, from flying gravel and what not from the road, as well as from above. Part of that idea was I needed to use the tarp for my floor for the yurt, so I didn't want to trash it needlessly.
However, I did of course wrap all the polyiso boards in the tarp. So the unwrapped plywood boards protected it from above and below, while giving the whole sandwich much needed support and rigidity against the wind.
I tied it all down very tightly with a long rope. The plywood boards allow you to tighten down the ropes well without damaging the edges of the iso panels. The tarp also helps in this aspect.
I did probably an average of about 75 mph hour most of the way there, this was on top of a volvo wagon, which had a rack on top that I tied all this to. The iso panels were just fine when we arrived at BM, and were in the same condition when we got back home. In good enough condition that I was able to sell my yurt for what it cost me (not including all the labor hours) in March.
You should be fine.
Burn on,
-Alejandro
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-Alejandro Moreno S.
GreenMBA, Dominican University of California