This is a bolt adjustable gt2 belt tension part. All holes are 7mm so 6mm to .25in bolts will work. It should work with all of the Corner designs that I have seen here in Thingyverse. The belt fits very tightly in the clip on my prints.
update 12-1-2015 : added a new connector rotated 90 degrees to align the direction to the belt
update: 12-6-2015 Moved mounting holes to align with corner blocks on part 3
Original wood frame Flashforge Creator
Everything was designed to avoid the need for supports.
All edges were rounded to eliminate most cleanup.