As you have rejected plywood as not being as strong, there are ambiguities with your question. Do you need flexibility of plastic for which plywood would splinter and break? Do you need compression strength, which would crack Lexan and possibly splinter plywood? Lexan polycarbonate or lexan acrylic? One is quite stronger than the other.
For inexpensive, consider salvaging discarded large-screen projection televisions. The housings are either particle board wood (unsuitable for so many things) or quite strong ABS plastic of 3 mm thickness.
These televisions are also constructed with a series of large panels over the rear-projection panel. Some are transparent acrylic. The rear projection panel is a frosted plastic and there may be a diffuser of 2-3 mm thickness as one of the panels.
Equally unsuited as a replacement will be a front-surface mirror of quite large dimensions. I add this because it's certainly a maker item to be salvaged for telescope or optical use, including kaleidoscopes.
The television housing will have some large expanses of flat segments, and also contain some angled portions which could be used for brackets and mounting components.