Summary

Tighten your Ultimaker belts
Parametric Design
Slim, fits in small space of Y belts
Adjustable tightening (via cable tie)
I have printed and fitted 4 on my machine today

Instructions

Try this one http://www.thingiverse.com/thing:24336 , It's better.

Print, they are small so use thin layers
The Scad file and STL file is small size for Y belts
You can increase height of Scad file for X belts
Place over belt underneath at opposite end to the block
Place flat side up for the Y belts
Rap 2.5mm cable tie around and tighten, make sure block part of cable tie is not on the flat side or it will scrape
Cut end off cable tie
Move belts by hand and make sure nothing is scraping
Start printing round circles again

Belt Tightener for Ultimaker