Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:2761298.
21.02.2018: Added V2 of the tensioner part (the green one). This has spacers builtin that only touch the inner ring of the iddlers bearings. I printed and installed this and it works as intended. The guide part also has V2 with the same builtin spacers but I had not printed and installed this. Should work though.
I wanted to fit toothed idlers and also add belt tensioner but did not like other designs ( too bulky, more extra parts, longer printing times,...) so I designed and printed this. It can be used as Ybelt tensioner or X belt tensioner without removing any extra stock pieces. Here I used my new toothed idlers but since their mounting/external dimensions are the same as the stock ones they can be used too. Extra parts: M4 nut M4x30mm bolt with hex head 2 D5x20mm pieces for iddler axes. Threaded or unthreaded metal studs or bolts will do.
I printed this in PETG (2.5h with my slow settings) in the suggested orientations. Could be printed entirely without supports but I think that it will be weaker that way. The FreeCAD file is also there should you want to adjust tolerances, change for other size bolts,...
If you make one please post your make and any problems/suggestions you encountered.