Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2569542.
An adjustable idler to be mounted using a 20x40x1,5mm edge profile on a 2020-beam. The idler has a holder holding the 2 flanged F625FF bearings, a guide to lock the direction of the idler and a knob containing a wingnut to adjust the tension on the belt.
Printing I printed this one in PETG using a 0,6mm nozzle and 0,2mm layers (could be thicker). Support is needed for the knob.
Mounting First insert a hex head M4x20mm screw in the bottom of the holder.. This should be a very tight fit in the cavity for the nut. Heating (and a bit of force) may help. the bearings are mounted on a M5x30mm bolt&nut.Put a spacer between bearing and holder on both sides. Bearings are mounted with flanges to the outside (of course). The guide is placed as shown in the pictures. Insert the wingnut in the knob and fix the idler combination on the aluminum profile.
BTW . I used a bit of superglue to fix the guide to the alu. Orientation is set by the small notch.
added the drawing for the alu mounting profile (scale 1/1)