(Phenolic plastic, like linen-reinforced Micarta, is a pretty good rigid choice). You can duplicate a base plate pretty easily, by chucking a dowel in your router, and using a solid-carbide router bit in a second router, with the same diameter as the dowel.
The trick is, to mount the base-plate material, form a center hole by plunge-cutting, then making a pin-router jig of router #1 with a dowel and the holed but uncut baseplate, and cut it with router #2 which has a fully-formed base plate.
You can put a collar on the dowel to guide an oversize circle cut, making a large circular baseplate.
I've done this with phenolic (to make a matching-diameter baseplate for a small router), but it should work with aluminum as well.