Do not use acrylic it is very brittle and will shatter easily if contacted. Thats even in warmish temperatures, just imagine what winter would do to that. I'd suggest Lexan (aka polycarbonate). Both of these materials are sold in sheets at my local Lowe's. Lexan is quite a bit more expensive than acrylic, but I'd hate to see you get finished go for a drive and have your hubcap explode. Also this would create a hazard on the road if it did happen.
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