Actually since the proposed use of the mirror is not an optical path per se but a reflector then a sheet of polished metal would be a durable and efficient front surface mirror. If you can't find a source for some good stuff try aluminized mylar or maybe cut up an old chrome plated ferrotype tin. Sheet aluminum is the easiest to use but tends to dull quickly unless coated to prevent oxidation.
FWIW I've used those thin foam aluminized insulation sheets that builders put under siding as a cheap and quick reflector and can be cut up and assembled as needed. The uneven surface tends to diffuse the light a bit better than a smooth one would.