I made something similar but instead of square aluminum tubing, I used wood, seemed easier to work with than metal. A good adjustable camera mount usually costs $20-30 alone. If you go that route and use a camcorder, make sure the mount is big/sturdy enough to support the weight of the camcorder and not start moving around given vibration and g-forces on track. But don't need to go overboard either.
But given the cost of all the parts for a DIY, for only $50, the CruiseCam mount sounds like a good deal. Isn't much more than the parts, and you avoid all the labor and effort!
Golden Gate BMWCCA - Car Control Clinics
'12 328i/'11 C2S/'04 GT3
Past: '09 M3 DCT/'03 M3 SMG/'95 M3... missing an E30 I suppose...