This bearing holder for LM8UU bearing is designed to lay flat on y-carriage with an additional zero vertical height. It consists of 2 brackets that enclose the bearing from the sides. The precision depends upon the parallel line up of the mounting holes in the y-carriage.
I recently ordered a prusa I3 derivative aluminium frame from a 3D printer shop in Spain. The machine drilled y-carriage is specified to have drilled holes for SC10UU bearings. In order to use my LM8UU bearings, I designed this kind of holder. The measures of the mounting holes are dx=28mm and dy=21mm. Use 4x M5 20mm bolts to mount the bearing holder.
Feel free to adjust the spacing in the OpenSCAD-file.
You can use sand paper to get a flat bottom of the holder. It increases the tension on the bearing, too.