This is a pretty simple but kinda time-consuming one day build, but you have to be careful, as it involves sharp box cutters and permanently modifying footwear, so be careful.

here are the files.

Supplies

Parts (per pair of footwear)

the footwear you are not afraid of cutting
4 abec7 bearings (2 per wheel)
2 rods or threaded rods (8mm thick, 38mm long)
3D print 2 wheels and 2 holders.

Tools:

box cutter, Exacto knife, or another sharp tool for cutting rubber and foam.

Step 1: Step1: Measure Twice, Cut Once.

you are going to need to cut a 20mm deep hole on your footwear, so make sure you have room to fit, otherwise, you will cut right through your shoe. then, find a good spot and outline with a sharpie, I recommend making a hole 2mm smaller than the outline, so you can enlarge it later if necessary. A new, sharp blade is a must, it makes things a lot easier.

42 x 34 x 20mm hole on footwear.

Step 2: Step2: Assembly

Press fit the 2 bearings, on both sides, and insert the rod, same for the other wheel, after that insert the holder on the hole you cut on the other step, and you're done!