This is a slightly altered version of http://www.thingiverse.com/thing:1160707 by WheresWaldo
I removed 1mm from the top and bottom, as well as removed the counter sync's and then elongated the holes so you can adjust side to side to remove binding. I had issues with binding on the original and my printer, It was probably my 8mm rods but I changed this block to suit my needs. I also eliminated the need for a 1mm spacer. This was printed with ABS with Scale set to 103% if printed with pla it should be fine as is.