Compact Thomson Super 8 Ball Bushing Block (bearing block)

Update 11/09/14: An even better design is here: http://www.thingiverse.com/thing:534210 or here: http://www.thingiverse.com/thing:540629
They use screws to precisely adjust fitment- no more cable ties!
For 1/2" guide rail, bearing is held in block with two tie-wraps, block has two holes for #6 screws on center line and spaced 26mm apart. The flat surface is 19.05 mm (3/4") from the center of the guide rail.
Use small tie-wraps to provide fine adjustment of "fitment". Put the bearing o