Belt tensioner originaly created for a modified printrbot.
Two differents belt type can be used: T5 or T2.5, download the dedicated stl file.
Add a M3 screw (30mm+ )
Put daddy part in mommy part
Add any M3 washer and nut on the scew
If your print is not clean or the belt doesn't want
to be inserted in the belt-hole use a screw driver to enlarge the hole in the same time, it work fine.
Update: tens-T(2.5/5)-40mm are 40mm long and the belt can slide a bit better in the hole...