I was trying to find an easy/fast way to mount various things to my 15x15 Open Beam extrusions. I downloaded a few things, but nothing worked that great for my application. So I ended up just modeling something that seems to work pretty well. Feel free to use or modify if you like. It has an m3 nut trap (I use needle nose pliers to squeeze the nut in to place) and of course an m3 bolt. I added a negative space on both sides to help pull off the Open Beam extrusion if needed. Without that, it's a pain to pull off...Modeled in Maya and Printed with Slic3r.