Work in progress. I really liked the initial D-Bot Core-XY setup http://www.thingiverse.com/thing:1001065, but there were a few things that I changed. I made it ride on both the back and the front rails and shifted the lead screws towards the center, added a part to connect the belts together so that z steps aren't lost, beefed up some of the parts, and changed the bed mounting brackets to something a little more rigid.
I added an optical end stop for the z axis because it is a lot more precise to alter the first layer z height in firmware than with a screw. There are a few things you need to modify in the firmware to use it. I have attached two photos of what you'll need to edit in the firmware. The first photo is changing the endstop to trigger when the visual field is broken. The second picture is where you adjust the manual home position. You adjust the z value to change first layer height.
Optical end stop: http://r.ebay.com/M1UBdX
The belt tensioner uses a 860mm GT2 timing belt and the same pulley system that is used for the belts above and m3 washers are used to get the right height. Two more GT2 16T pulleys will be needed to attach the belt to the motor shafts. You will need to obtain stepper motors with longer shafts to attach to the coupler whole accommodating the pulley. Mine are 22mm in length.
I used countersunk m3 screws with a 3 point leveling system to level the 1/4in cast (NOT extruded) aluminum bed. Cast aluminum is perfectly flat, extruded is not and will not work for a bed this size.
I used a laser cutter to cut out the triangular brackets. I provided the SVG files for them.
E3dD V6 Fan: http://www.thingiverse.com/thing:354723
V Slot Covers: http://www.thingiverse.com/thing:832077