I thought I had a solution. Got the SRAM 15mm conversion caps. Wasn't sure if they would fit, the description only had the diameter. When I received them, and compared, I didn't think they would work as the sizes were off. Then I tested it on the bike to see, and it actually fit! It kept the wheel in place and ended up being the right total width of 100mm. However there was a pitfall, it wouldn't let the wheel spin... I consider that kind of a deal breaker.
Original on the left. I could tell the external diameter was too large to fit in the hub.
Even if it did fit, it was not the right height, so it would have been to short.
However, not fitting in to the hub actually made up the difference. But alas, no spin.
Maybe I can modify it somehow... Or do the pvc suggestion. Work in progress.