A long time ago (in what feels like a galaxy far far away), my high school buddy built an Newtonian reflector using a concrete form tube from Home Depot as the main tube, spray painted matt black:
Back then, he hand grounded a (spherical, I’m assuming) mirror. Today, if I were to do this, I would go the route of spin-casting a parabolic surface by spinning a tub of epoxy resin on a constant rotation speed and waiting until the surface sets into a parabola, then silvering the surface using Tollen’s reagent:
From what I know about optics, I would suggest putting both the mirror and the raspberry pi camera on kinematic mounts. (See: http://www.thingiverse.com/thing:1063431) for example.