Printer bed support

This is for use with SonnyLowe's Y mod for the Folger Tech 2020 Prusa i3, which you can find here: http://www.thingiverse.com/thing:1278055

I am using this plate on top of the openbuilds Y gantry plate to add some rigidity to the rather soft folger tech aluminum plate. In spite of the added thickness, the 2080 rail mod lowers the print bed slightly, so no overall print height is lost. This plate adds enough clearance to allow the full 200mm range of movement even if the rail is only as long as the printer frame (405mm). The added thickness also insures the bed doesn't hit the endstop before the gantry. The large holes on one side are slightly larger because the screws on one side of the gantry move in the elliptical mounts as you tighten the wheels to the rail.

For V3, I reduced the fillets in the inside corners and made the center part wider than earlier versions, which allows for more endstop mounting options.

The V4 file has recessed holes for the mount to the gantry