After seeing the remix of this http://www.thingiverse.com/thing:981260 , I made a rev2 version of the grooved clamp to incorporate this idea & made the hole and edges round. I also added some welds around the clamp to make it more solid.
Here are 3 different options for a belt clamp to use instead of 1 of the zip ties at the corner blocks or attach to hook on http://www.thingiverse.com/thing:853977.
I am starting to put this on my machine now & the 2nd & 3rd photos show it attached using the standard method & using the Mechanical Belt Tensioner option. It does not look like it will work well with the Mechanical Belt Tensioner as that clip hits the bearing on side that gets closer to the end. The long version clamp is probably too long to use with this, but included it in case it is useful for some other project.
These clamp designs originated from http://www.thingiverse.com/thing:627214 & http://www.thingiverse.com/thing:643799
You will need 8 of these for the Mostly Printed CNC project. I would print one of each 1st to see which works best for you Print either of the versions you wish to use. The small clips only weigh about .04oz each.
One of the designs is printed in pairs so the belt will be going the same way thru the clamp on each end.