The two main materials used for my second R2-D3 are styrene and resin. Both of these materials have their quirks, but they are easy to work with and inexpensive in comparison to aluminum. Also quite a bit lighter, though some of the resin parts can get a little heavy.
From beginning to end, the build time on this project was approximately 18 months. There was a lot of extra time spent on building sections of the droid that would have normally been handled by a machine shop. For example, the legs were built out of sheets of styrene as well as several other sections.
All of the resin parts and some of the styrene elements (dome, skins and frame) were purchased from approved suppliers from Astromech.net. Several different vendors were used and all of the parts were exceptionally well made.
Almost every exposed surface on this build is painted. All of the painting was done using "rattle cans", most of which were purchased at Lowes and Auto Zone. Painting is a huge part of any Astromech build, especially a non-metal Astromech.