Short of googling acrylic windows, which I'll assume you've already done, here's
a couple of threads about finding custom windows from on here. Have a browse when you can.
There are some success stories therein, if you don't mind a bit of reading.
Might there be other options besides aluminum or ss? How about tempered glass with a stone
blocking screen of some sort over it, to give it a chance against stone chips? I believe you can
get it custom fit, although it's not cheap.