Carbon fiber will be the lightest material but will be the most expensive. I think the only way you will get them is to make your own by buying the cloth and using an epoxy such as West Systems. The cloth needs to be wrapped over a form, usually a hard styrofoam, which is slightly smaller in diameter than your final dimension. Also carbon fiber needs to be layered in different directions (like plywood) for maximum strength, i.e. 0degrees, -60 degrees, +60 degrees, 0 degrees, -60 etc. for at least seven layers and possibly more. Experiment first, but my feeling would be that it should be thicker in the middle, tapering off toward the ends, or at least thicker (or larger diameter) at the mirror box. Each layer of carbon cloth and epoxy needs to set before you can add the next one. If timed right, the next layer can be put straight on, but once set hard, the layer should be wiped down with acetone and then sanded to give the next layer enough tooth to hold.