Square aluminum tubing. Interesting how that works, huh?
Actually doesn't come with holes, I just drilled those on my own then powdercoated the whole thing. You can get it at Lowes or Home Depot. Below is a link to mine.
piknockout's 2003 Nissan SE-R on CarDomain - Page 6
Seriously though, I would save yourself the hassle and just buy one of these. Can be used on multiple cars and adjusted to your liking. And only $50, as stated you'll spend somewhere between $25-$40 doing one on your own. Save yourself the hassle.
CruiseCam Headrest Camera Mount