End result image posted first.

I wanted to take an existing lightweight shelf rack I had and put wheels on it in order to make it more portable. The shelf feet have a hollow body like in the pictures, so I went to Home Depot and Lowe's to look at casters (wheels that swivel 360 degrees) and found 2 basic designs.

1) Drill a hole into the foot of your object to insert a pin or screw in.

2) Mount the caster with a plate and some screws.

Option 1 was much cheaper and simpler. And armed with a 3D printer, my mind immediately went to think that the feet cavities just need to be filled to insert the caster pin into. Then instead of drilling a hole which the instructions ask for (designed for wood/metal objects like a desk), I could just print it with the hole already designed in. So I bought the cheapest I could find, a 4-pack of the Everbilt 1-5/8 in White Plastic Stem Casters. With this design in mind, the casters, a 3D printer, TinkerCAD, some calipers, and my subject to enhance, everything was ready to go.

Supplies

Casters (4-pack of the Everbilt 1-5/8 in White Plastic Stem Casters)
3D printer (Ender 3 Pro)
Filament (Transparent Gizmo Dorks PLA. Any would work, this is just what I had loaded)
Calipers

Step 1: Measure All the Things!

For the hollow shelf feet, everything was circular so that made things pretty simple. It was straight on the top side that sticks into the shelf, then expands outward.

The smaller inner diameter was 1.3in and the widest part of the bottom foot part was 1.8in. So I had an idea of making the adapter a cylinder just smaller than 1.3in, and the bottom to be 1.8in to catch the bottom of the foot.

Then the 5/16in caster pin stem from the package in decimals is 0.3125in.

Step 2: TinkerCAD Design

I started the wider part using a paraboloid shape, diameter 1.8in and an arbitrary height of 1.25in. It looked to create just about the right taper shape as the foot.

Then I added the (orange) cylinder in the middle with a diameter of 1.28in, just incrementally small enough for 3D printer tolerances and make sure it fits into the hollow straight cylinder part.

Then finally a cylinder cut in the center the entire object with diameter 3/8in, or 0.375. Since the shelf would always have be loaded with downwards gravity, it could just be larger than the minimum 5/16in stem from the package and it would stay put in place. My pack of casters also came with press fittings, so I had a quite a lot of tolerance here to play with here. Worst case I could glue the pin in to help with setting it up.

Step 3: Printing and Test Fitting

I sliced the object in Cura with a 50% rectangular grid infill. I just used the PLA already loaded on the printer.

After printing, the caster fit into my printed adapter very snugly with the press fitting, so that was perfect. The adapter did sink into the foot deeper than I wanted...but it still swiveled clear of the foot. The curvature was about just right and pressed up against the foot curvature pretty evenly. By luck, if I pressed the adapter into the foot with a little force, it actually stuck a little bit, so that made installing onto the shelf that much easier, but could also very easily removed. So overall I was happy with it.

Step 4: Installing and Closing Thoughts

I printed 3 more of the same thing and installed them onto the shelf. Works really well!

Since the design works, fits very well, I can't think of any glaring failure points, I haven't improved it at all. I could only think of possibly playing with the paraboloid parameters more so that it mates with the foot lower and make it more flush with the feet. Since it's just an aesthetic improvement, I've decided to just leave it as is. Then as for the PLA, I think the 50% infill is solid enough for the lightweight shelf. These adapters will be my experiment for how long can PLA last in a light-structural application. From what I've read, PLA is great for ease of handling and creating models, but when using for structural applications other materials like ABS or PETG are highly recommended.

There's so many creative functional things that a 3D printer is capable of. I also learned at least a bit more about casters while I was at it, something that I imagine never gets any detailed attention. I'm overall super happy that a cheap, lightweight shelf was able to be upgraded with even more utility with little effort.