It is 1/4" that is used to mount the motor, and the base is 1/2". The 1/4" is really stiff, especially when it is only 2.5" of it, but the problem was mounting it to a base that was thick enough to screw into, so 1/2" fit the bill.
Rather than getting huge sheets of it too, I found an online resource,
www.estreetplastics.com. They sell smaller pieces, 6" x 12" if you only need a little bit of it. I still have some leftover. Took about 2 days to ship, then it came in the mail, easy transaction.
I prefer Lexan over aluminum after this. Key to Lexan/Polycarb, DO NOT USE LOCTITE. The Loctite will eat right into the Lexan and ruin it. So if you screw into it, make sure it gets a good bite, and if you can, use a nut on the other side to hold it, rather than rely on the threads. Lots of bracing to avoid bolts pulling out.
Also, for people that don't know, Plexiglass is different than Lexan/Polycarb, much more brittle and not really tough for what you'd want in an RC application. While it is cheaper to buy, don't be fooled.