Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5147697.
Important notes!- Update April 2022 1-Don't use epoxy. Use Bob Smith Cyanoacrylate or Weld-on 3 (prefered). I weight tested these wheels with 240lbs of salt for my purifier and it did pretty good until I hit the rounded curb and the wheel turned sideways and blew apart.(Damaged was expected) When i inspected the broken wheel the epoxy simply peeled off in a layer. I tried the same thing with Weld-on 3 and it Welded the plastic (PETG) together and removed chunks from each half, a much stronger wheel. 2- Use axle spacers to take up the side to side play on the axles. The bearings are a good fit but after continued heavy use they will work out of the bearing races. You could also try superglue, but be careful not to get it into the bearing.
Remixed an excellent design and added a bearing race to improve load bearing and lower rolling resistance. The only bearings i could find that fit the application somewhat close were these i found on amazon. https://smile.amazon.com/dp/B08F5HVFQG/ref=cm_sw_r_tw_dp_F55NSVC772ZE6Z8DMD0D?_encoding=UTF8&psc=1 12mm x 28mm x 8mm bearings. The axle shaft is slightly smaller than 12mm but its not a machined surface so there isn't a bearing that fits exact AFAIK. The fitment will still be tighter than worn plastic wheels. I print my wheels in PETG with enough walls to make the spokes solid. I am using .28 layer height and a 0.6 line width. These wheels are printing in about 2 1/2 hours for each piece. I'm using a bolt to line up the wheels and epoxy to secure them together, tighten a washer and nut down on the bolt to hold them together for drying. Good luck, Enjoy. Note, I have printed two sets but haven't assembled them yet. If there are any changes needed i'll post it. EDIT- All 4 assembled and on the wagon, they fit awesome and roll soooo smooth. Video to come! I opened up the inner part of the wheel (using a drill)slightly larger than the bearing race to free up the wheels for an easier pull.
The bearings are a perfect press fit with my fingers. If you use PLA it will likely make the hole too small so you would need to adjust the wall inset to get desired results.