I'm glad to announce VERSION 2 (RED PICTURES).
1/ The back side of bearing block is a tiny bit smaller. Now it does not touch the Prusa frame when the carriage is lifted up to the very top. From now your printing height limit depends not on the carriage but on the nut on the leading screw.
2/ There are several sizes to avoid play. The key to avoid play is the distance from the center of the X-rod to the center of the bearing. Ideally is is 9.00 mm. Other versions are more tight. There are versions:
X_bearing_block_v2_9_00_mm_left = distance is 9.00 mm
X_bearing_block_v2_8_95_mm_left = distance is 8.95 mm
X_bearing_block_v2_8_90_mm_left = distance is 8.90 mm
X_bearing_block_v2_8_80_mm_left = distance is 8.80 mm
X_bearing_block_v2_8_50_mm_left = distance is 8.50 mm
In my case version with 8.95 mm is best. In most cases one of the first three should be OK.
3/ You may mount it one bearing up and two down or two up and one down. They will function equally well.
4/ I have finally (after 18 months working with looong breaks) finished the compact Sunhokey X-carriage for chinese E3D V5 clone. It is just fantastic. Here it is:
INITIAL DESCRIPTION (YELLOW PICTURES):
The big problem to solve: shaking X-carriage. Here is video of the problem:
The small problem to solve: make X-carriage run faster and smoother with less resistance.
First idea to use 623zz bearings came from tahustvedt:
But he reports that the bearing block develops play (luft). I suggested it's because of "non-ring" design. But the "full ring" or "full square" design does not fit: it touches belts.
So, the second idea came from:
Putting bearing at the 120 deg angle saves some space and allows "full-circle design".
As a result I propose a bearing block with three 623zz bearings (outer diameter 10 mm, inner diameter 3 mm, width 4 mm), which are very cheap. You will need three M3x16 screws. The whole thing does not touch belts. You can mount it to the vertical X-carriage wall with the stock M4 screws.
Location for mounting: you may use two blocks instead of one LM8UU. To make it more stable orient blocks with the bearings looking outside. Wider you place the blocks more stable will be your carriage. At first you may attach it using stock holes for M4 screws. When you will create your own X-carriage you may place the blocks as wide as you like. If they are wide enough on upper shaft you may place only one on the lower shaft.
IMPORTANT: Installation is very simple: you only need to unscrew one M3 screw on the left side of your printer. Than removing one plastic part you will be able to pull out half of the shaft. Than you remove LM8UU from it and put the block on the shaft.
The sequence of pictures shows how to install them.
P.S. This is my first CAD experience. I'm open to critics and improvements.
P.P.S. Here is similar design for Y-axis (table).