I put black ABS skirts on the Wagner Roadster and we found that the stuff grew in the sun/heat. It buckles and gets wavy, and does NOT shrink back to original size. It worked well, and bolts on nicely....just gets wavy. I had to add aluminum angle behind the longer runs to keep it from opening gaps at the bottom edge of the body. I used the thick stuff (I think it was 3/16 or 1/4).
One thing that did happen, was that I set it up as a tunnel under the car (closed at front, open at rear of the 27T body). That gave us problems with the car sucking the belly pan downward, under the engine (that area was a separate pan, closer to the salt because of pan and bellhousing sizes). In 2000 it sucked the belly pan down far enough to buckle and crack it. We were open on top of the engine, so I guess the pressure differential was enough to cause a problem.
food for thought,
regards, JimL