While sourcing parts to make an idler pulley, I felt forced by price to buy more parts than needed. So I have a few for sale if anyone is interested. $12 shipped for 2.
It uses an M6 barrel nut as the shaft for the two 698 bearings. For clearance, the bearings are slightly smaller than 608 skate bearings at 19mm diameter. It comes with 3 speed rings to space the bearings, and 2 washers to adjust spacing to mount. Drill and tap an M6 hole in your mount and the barrel nut secures the other side. Loctite recommended.
The bearings are sealed 6mm wide 19mm diameter with 8mm bore. They can be replaced with 608 bearings if wanted. The barrel nut is 18mm tall and has 15mm of usable shaft. Compatible with 12 and 15 mm belts. Low profile, will work with larger wheels.
Here are some pics, not fully installed yet, waiting on belt.
Let me know if you would like one, I will ship out immediately.