Thanks for the comment. I put belt loop before putting XY rods into bearings and then stretching the belt using rods. For some reason it works.

Looks like this could be useful for people with Ultimaker 3s if they somehow find the need to replace this part or use an open belt. Quite an elegant design, meshing the belt onto itself. It does make the tightness a bit coarse, but I guess someone can use it in conjunction with one of the other springy tensioners available on YouMagine.