These are glass clips / holders for mounting an IKEA SÃ¶rli mirror (20x20 cm, â‚¬5.70 for 4, article nr. 600.740.07) on a Printrbot+ heated printbed.
I made these to secure my glass printing surface (IKEA SÃ¶rli mirror with a sheet of Kapton tape) to my printbed. I use the heated PCB directly on the Plywood surface, I found it too hard to get it flat with cork underneath.
I also found that my printbed/PCB were not completely flat. Therefor I let all four holders cover the mirror, such that when the screws are tightened, they force the PCB and plywood to be flat under the glass. This decreases the printing area somewhat (about 8 mm of the left and front so that the home position won't cause the hotend to hit the front left holder. Initially I made a version of the front left holder without a covering part, but then my mirror would not stay fixed, and the mirror would not be parallell to the print surface. I adjusted my X and Y end stops such that the home position would not hit the front left stop.
The SÃ¶rli mirrors have rounded corners, which these holders should fit perfectly.
The front holders have pockets to allow them to fit onto the screws holding the printbed to the 'boots' attaching it to the Y rods.
The rear corners have nicer OpenSCAD source code, since they were written after the first ones. They should be easy to modify.
Update 2012-12-29: Uploaded new versions of Front Right and Rear Left flipped the right way down for printing. Also rounded the holes a bit more in one of them.