To make the sub-wheels I had to make a mold, unfortunately I do not have any footage of the mold being manufactured, but it was also made on a CNC mill. Here is what it looks like.

The round aluminium bar in the mold is the center of the rollers. It used to be a solid 3/8 bar stock, but I cut it to length, and drilled a 3/16 hole in it. The ridges in the aluminium is done on purpose so the non-expandable polyurethane is less likely to slip off.

Here is a video of me making the sub-wheels​. Each wheel required 15 in my design, with four wheels that is 60 total.

In case you can't view the video here is the final product. The process required mixing the two parts of the mixture for the non-expandable polyurethane, in a 4:1 ratio, and slowly pouring the mixture into the mold, to let air out. Then I waited for a couple hours(time depended on the temperature, on hotter days the reaction was faster). Once the reaction was finished, I took the wheels out of the mold, and cut the access.