Bushing blocks fits to standard 11mm outer diameter and 12mm long bronze bearings.

I used the belt tensioner idea from user MoonCactus (with minor changes):
http://www.thingiverse.com/thing:45236
For tensioning your belts you can use his bananas.

Second picture (mounted block) was taken after about 8 months - still works without any problems.

For use in heated chamber ABS is recommended.