Summary

This Y-carriage part is printed as whole part and is designed with bearing guide to keep T2.5 belt straight as possible in standard Prusa Mendel v2 model. One of the biggest things.

Instructions

There are many Y-carriages:

as 4 separated pieces for LM8UU / 3 separated clamps for belt.
as whole thing in one for LM8UU + 3 clamps for belt.

Now, i have used http://www.thingiverse.com/thing:29744 in my printer, but it has some problems. It seems a bit too low ~5mm which the belts over 22mm bearings are not straight at the ends.
And i want add bearing guides because i don't like default method for Y-carriage belt. The belt cannot keep on the bearings and will always go a bit to left or right, bumping against the washer. That cause some friction and the motor must work harder. The risk that motor miss the coord is greater.
And the bearing guides makes bearing wheel a bit thicker from 22mm to ~27mm.
Thus the belt line is a bit higher, so the Y-carriage plate height must a bit raised to avoid bump against the bearing guide at the end and make the belt straight again.
That is why i redesigned the Y-carriage part again to fix some problems.
And i don't like the LM8UU clamps in used Y-carriage part. It will break easy off. Mine has less risk for breaking off and with screws it will keep LM8UU bearings better on place.

If you want use the new Y-carriage, it's recommened you use bearing guides too. I know there are many bearing guides, but i have just re-designed mine.
I have designed like http://www.thingiverse.com/thing:28683, but mine has raised inner diameter from 22mm to 23mm (in Sketchup) because 0.5mm nozzle printing will reduce the actual diameter a bit. I have tested it with 22mm bearings and it fit exactly in 23mm version.
There are 2 half versions for bearing guide. One with 4mm and another with 3,5mm. You may choose what you want, but i used 3,5mm+4mm for my bearing, and it fit exactly enough. You need only glue both together (aceton as glue is okay).
The bearing has 7mm width, but the printing don't make it exactly 7mm room because fill room of 0.5 mm nozzle. So 3,5mm+3.5mm will not fit it exacly, there will be little room.

The Y-carriage is 185 mm in length, thus most printers will print it. But the outer line might have problems.... I hope you can print it without much problems.

You will see the belt part is not exactly in the middle. It's a bit moved to make sure stepper motor + belt has no problems with it. And this Y-carriage is a little smaller compared to previous carriage.

Until everything is checked, this project is still work in progress.
Good luck with printing!

Custom Y-carriage and bearing guides