Here I am answering my own question. I found a 2'x4' sheet of 1/4" plexiglass through the local classifieds for dirt-cheap. I traced the shape of the rear rollbar onto it, cut it with a jigsaw, and sanded the edges with a rasp. Along the sides and top, I used a rotozip to cut 7 evenly-spaced "slits", through which fed straps of industrial velcro to attach the plexiglass to the rollbar. It worked fine but felt a little loose. So I ended up running a bead of clear silicone around the entire edge, where the plexiglass contacted the rollbar. That part is not beautiful, but the top and side edges are all covered by the fabric cover anyway, so it didn't need to be. I've had it on there for a couple of months and it's worked out great so far.