Seriously, I would discourage you against using plexi. You'll be replacing it within a couple years. Plexi is fairly expensive and it WILL scratch, no matter what you do to try to keep it from scratching. I strongly suggest using tempered glass with a solar gray (#16) tint.
I'm not sure where you guys are but onedayglass.com is great or find a local glass shop. Odds are the glass is going to be cheaper, especially in the long run, than plexi.