your local building supplier may be able to supply the plexiglass, and you can drill the holes yourself with a bullet bit from black and Decker,just remember to put a back up board behind where your drilling, also LOOK in the phone book under glass to locate a glass comp-  locally in your neighborhood..

I prefer Blue Ridge Hobbies in Greenville SC, great prices and selection,they ship quick contact at www.blueridgehobbies.com....love this store. Frank is knowlegable and customer friendly give them a try.....steamchugger

I prefer mdf  (medium density fiber board)  to plywood,it will not warp or distort,you can get a 1/2" sheet 49"x97" for about $20.00 bucks plus tax from home depot or lowes. I also lay cork roadbed to absorb the noise, works well, can be used easily.nailos and spikes hold well.

try googling or other search engine for plywood/custom sizes i know they make different sizes its just a matter of locating one and also wether its cost  effective