These 12volt or 24volt motors are ubiquitous on AliExpress and DealExtreme. The only mounting holes they have are in the face end, which means they need some kind of bracket to hold them.
This bracket mounts the motor in a sleeve that can be rotated to adjust belt tension. You may have to adjust the clearance or skim the parts in a lathe to get smooth adjustment. See the original file for a single-partd non-adjustable version.
This bracket uses M3 screws to hold the motor and M4 to hold the bracket.
All measurements are customisable in the SCAD file.
I suggest wall thickness of 1.6mm.
I turned the parts on a lathe to adjust fit. You could also adjust the clearance value in the SCAD file.