This is a couple of pieces that bolt to a Prusa i3 Y carriage and allow a RepRapPro heatbed insulator to be bolted on.
So I really like the mounting system for the RepRapPro heatbed (http://www.thingiverse.com/thing:20968 and http://reprappro.com/wiki/index.php/RepRapPro_Tricolour_heatbed_assembly). It bolts an aluminium top surface, the PCB (a MK2 style PCB bed) and an MDF insulator together, and then the insulator is mounted to the Y carriage with a three-point, spring-loaded mechanism.
The three points makes for easier levelling, and I prefer to keep the structural load away from the PCB itself. And I also like to have some insulation there, and an aluminium heat spreader is great. RepRapPro sell the kits separately if you get in touch with them.
Problem is that the standard Prusa i3 (https://github.com/josefprusa/Prusa3) design expects the four corners to be used to mount the heatbed, so there are no holes (or material where I could drill some) in the places where the RepRapPro heat bed wants them. So I designed these extensions to allow the heatbed to be mounted. They are simple, and means sacrificing a bit of z height, but will do the job until I can get a modified Y carriage cut.
They use the same holes as the zip-ties that would normally hold the y-carriage bearings, but I already used those holes to bolt in bushings to hold the bearings (http://www.thingiverse.com/thing:94256).
Eventually I will merge the extension with the bushings, that makes sense.