OK easy part first. The base is unistrut, because it's easy to work with and tough. I bought a 10' piece and cut it (er, my friend cut it) into a 4' piece and two 18" pieces. You can buy the channel (long pieces) at Home Depot or the like. The fittings you can buy at plumbing supply stores or order special pieces on-line.
Here's a close up of where the channels intersect. There's a X-fitting which my plumbing supply didn't have so I used a T and a two-hole piece.
You can see a nut where the 1/2" rod goes through the cross, there's a matching nut on the underside of those metal fittings to keep it tight. Also, the nuts you see going into the fittings are coarse 1/2" made by unistrut, they mate with unistrut bolts with springs.
Cost for the 10' piece of channel was $22.70, the T fitting was $12, nuts and bolts $5.40. So, $40 for unistrut. Too expensive really but OK for prototyping. If this were a larger implementation, say 10-20 mirrors, it could be made into a grid so as a system it could be stiffer. To cut costs, smaller channel pieces (13/16") could be used, or maybe a cheaper, plastic substitute. Also, the L and T pieces are pricey and in a grid it might be possible to use flat 3-hole pieces. All in, $20/ device (per square meter) for an easily installed base might be the target.
The threaded rod was a 2' piece of 1/2" -13 from Osh. This costs about $1.50/foot. It would be better to cut it down to 1' for stiffness (the mirror doesn't require 2' for clearance), but leave it at 2' when first building -- it's easier to get under it to work.
Here's a pic of where the 1/4"-20 rod is coupled to the 1/2". The 1/4" isn't stiff enough to use for the whole length, but 1/4" is needed to fit through the hollow shaft of the servo gearbox. The 1/2"-13 to 1/4"-20 reducer coupling nut is an oddball. I found it at tannerbolt.com, here's the link. I had to buy a box of 50, but they're 50 cents each in quantity. I have 49 left. The bolt you see in the picture is what the gearbox will rest on. Will tackle that in a next post.