Square dobsonian

[Naemeth]

I have wondered if there are alternatives to glass for mirrors and it seems there is: plastics, ceramics, metals other composites and even liquid has anyone tried any of these?

You can use almost anything for a mirror. People have used gold, obsidian, clay and plastic to name but a few (see Unusual Telescopes by Peter L. Manly). Glass is best though as it is cheap and relatively light, and good at operating at low temperatures.

Back to the original question, you can use any shape you like as long as it doesn't obstruct the mirror .