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
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.