Readily-available plastic with high softening temp?

I want to make a holder for a GPS in my car. I'm concerned that in the summer the temps (think: parking lot, direct sun, windows up) will soften the
plastic and (with the weight of the GPS) cause the piece to do a "Salvador Dali" (for those who don't get the reference, Google "Persistence of Memory").
What plastics can withstand 200F temps yet can be heated and formed in an domestic oven (say, under 400F)?
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