I actually use Hydrofarm reflective film: https://www.amazon.com/Hydrofarm-MY2M25-Reflective-Metalized-Film/dp/B007A2WR1Q
I tape it to cardboard to make "panels" that I can remove. It does get scratched up but I only paid like $15 for a 4 foot by 25 foot roll so I have plenty for replacements, lol.
But definitely a good tip for anyone else reading. Just use some white posterboard, much cheaper and available just about everywhere.