Interestingly, vacuum-backed mylar has been used for light 'DIY parabolic mirror' duties. Here's one made out of a trashcan lid:

https://www.youtube.com/watch?v=_8sd9UgjXLE

I'm sure that durabililty would be an issue, and the vacuum would probably leak over time without some significant effort, and it probably wouldn't be very stable under environmental changes, but it's a cool lateral solution.