Looks like tinfoil - which is fine.
I would be more concerned with the thickness of the Plexi. Even at 1/2" its still not strong enough to combat the intense flex that even a moderately sized sub would make. 3/4" is better. I am assuming you will be using 3/4"

What is the box tuned to? What kind of Orion subs and what kind of power will you be using? I'm an avid enclosure-meister myself so this kinda stuff interests me