1/4 polycarbonate lexan sheet 11.7/8 x 48 - eBay (item 130157612514 end time Jan-18-09 16:56:58 PST)
If you can't find anything locally, I bought my 1/16 stuff for the aero on the Intrigue from this seller. Go with Lexan if you can at all, since it is less fragile than plexiglass. I cut the 1/16 with a pair of Wiss aviation yellow-handled snips, but 1/4 would have to be jigsaw/blade cut. Drilling the holes for mounting was also less messy than drilling the plexi I used for the windows in a certain car so many moons ago...
I haven't tried tapping either, but if you can't, maybe something like a sheetrock corner around the outside...that can also be used to bolt it down???