Acrylic or Plexiglass would be my choice. It's not as impact resistant as polycarbonate or Lexan, but doesn't scratch as easily, so it would be easier to see through for longer. But really, either would work.
You can find both at most hardware stores in the window and glass section. Or you can buy them online. The bigger the base, the thicker you'll want it to keep it from flexing too much. They also make premade router bases that you can buy at woodworking stores or online. You'll pay a bit more, but save some time. Though, if you've already got a router with a spiral bit and a drill press, it wouldn't be too hard to make your own. Both acrylic and polycarbonate work pretty easily, so long as you keep your speeds down so you don't melt the plastic. Just don't remove the protective film until after you've done your work to help reduce scratches.