First thing we needed after we received our Ultimaker was long belt tensioners. This design is modified from the wave tensioner by Gjis. As we tried thier design and found it was hard work to install and interfered with the case. We made some changes, such as making 1mm slimmer, that stopped interference with our ultimaker case. The cut outs in the channel allowed us to get a better angle when installing..
We think ultimaker should improve the belt adjustment in the design, until then, Maybe it will help with your machine if your belts need tensioning.
Ps, Photoed parts are a little rough looking as we printed them in approx 5 min.