Darn, I was hoping you'd tell me I overlooked a cheap prefab solution.
I did a little research on plexi(acrylic) vs lexan (polycarb) it seems lexan is the safer bet due to its impact strength.
I found this site
that sells custom cut lexan sheets. The lexan comes at 15% premium over the plexi. If I order five 40x18x.25" sheets I'll be set back $214 which includes $43 in shipping. They have a 1/8" thick product for $193. 1/4" looks like the better deal to me. There is also a 5% promo code if I "like" their facebook page
HD has 48x36x.22" sheets for local pickup priced at $67ea. I would need 3 of those, $201, plus I have to cut them myself.