This is the single most expensive piece of the whole system, it is also the most important. You need to use something straight and strong, for this I used something that I think you can get in any decent sized city and easy to find online. I used 3" x 1.5" x .188" aluminum channel.
There are 10 holes to drill in the rail 4 for the timing belt holders and 6 for the tripod risers.
Lets start with the risers.
The risers are needed so that the dolly does not hit the tripod head below. I used 1.5" x .5" flat bar aluminum 2.5" long. I marked the center for the tripod hold and the other two holes for mounting to the rail. Drill and tap the holes with * bit and 1/4 20 tap
Find the center of the bottom of the rail. I offset the riser 2.5" from the edge of the rail. mark the outer two holes from the riser and drill with a 1/4" bit. do the same on the other side and the center of the rail to give you 3 tripod points.
The placement of these is very important. you have to line them up with the idlers underneath the dolly so that the belt is straight. once you figure out the location simply mark the holes and drill them with a * drill bit and cut the threads with a 1/4 20 tap.