I've seen a few installes at some shows where they put a board/plexie over the subs/ports. Whats the purpose of this, and is there an advantage doing this?

It might be a mini loading wall. I will try to look for the pix of the inside of the monster Pioneer truck that had a loading wall. It keeps better back pressure on the cone (which I think someone told me about how it helps with impedance rise but I cannot remember so don't quote me) but it still allows air flow around to the mic.