I designed these rollers for use in a drybox, but can be used as stand alone.  They use 608 skateboard bearings.

The rollers are slightly crowned so that the spool will stay centered on the roller.

They are completely snap together.

I have printed these in both PLA and PETG.  

The rollers should be printed flange side down with supports enabled for the bearing cavity.  

Infill can be as low as 10% with no issues.

So far I have about 100 hours of use on them with zero issues.