The version here is for 15mm wide belt, the main body is 50mm diameter. Everything is parametric. The idler surface is crowned to help the belt stay centered. 
There is a flag to set that will print a small test to see if you have the OD on the bearing right. It should take a little force to insert the bearing. Print that first, adjust Bearing OD in the SCAD as needed.
The hardware stack is set up so that the nuts can be fully tightened without stressing the bearings (the spacer in the middle takes the force). 
 There is a bridging layer that needs to be cut out before using.
The hardware stack goes like this:
- Spin on a nut, put on a washer, then a bearing, then another washer, another nut. Tighten down. 
- Put the printed idler body on. 
- Follow up with another washer, a bearing, another washer and a nut. 
- Tighten the whole stack.