This is the one I used,
http://www.ebay.com/itm/Comp-Eng-20...t-/390464385713?forcev4exp=true&forceRpt=true
It doesn't come with the frame brackets but I used a holesaw and cut through the rails and welded the sleeve on both sides of each rail. This way it can be removed if needed. With the frame brackets welded in you can't remove it.