Matboard coated with acrylic medium works nicely. Plexiglass is a brand
name, so other sources of acrylic sheets are varied and usually cheaper.
I've gotten good prices on the acrylic used for screen doors at the hardware
store. It's a little thinner. I would think a glossy contact paper on
matboard would also work. Acrylic is one of those good things you can ask
your local framer to save scraps of.
But if you manage to get nice plates to begin with, you'll always have them.
It doesn't matter how scratched up or cloudy they get, they can be used over
and over. The scratches can be good. The issue of the plate being
transparent is certainly a useful part of learning the whole monoprint
process and to lead in to layering colors in printmaking.