Fabrocating groundskirts

I think we all know about the begining of the ground effects era, when racers used sliding groundskirts to seal the sides of their cars aginst air migrating underneath their cars. How did they keep them ridgid enough to hold their shape aginst the pressure differential and flexable enough to not deform on impact? Does anyone know how they were constructed (diagrams would be really nice), or any tips on how to build them?

Rubber.

I thought that they had something to slide on? It seems to me that rubber would wear down quickly like that, especaly at 200+mph