This is a modification of http://www.thingiverse.com/thing:30491
I added a bolt to hold the rod in place, fixed the nut orientation, and made it customizable.

The included STL is for 10mm Makerbeam, 8mm smooth rod, and M3 nuts and bolts.

This is part of a system of prints that turns Makerbeam into an optical rail. An optical rail is a long, straight, sturdy rail onto which optical components such as light sources and lenses can be bolted down and easily shifted along the length of the rail.

Commercial optical rail sells for $250+/m. Makerbeam only costs $12/m for the fancy black anodized stuff. You get the idea.

This component is inspired by the Open-source optics project, whose goal is to radically reduce the cost of scientific optical hardware, by the Michigan Tech's Open Sustainability Technology Research Group.

UPDATE: now slightly smaller, using measured sizes (and some leeway) for the nuts to reduce slot size. And tighter on the post/rod. Nuts are now optional (can use nut in rail slot).