So I went on a mission. My vision(as you likely anticipated) is for round corners, soft and flowing. So I started looking at end caps for pipe. My first prototypes were from metal pipe. They looked good but super heavy. All I could think of is one of these things falls off when I hit a bump on the road and a poor passenger gets knocked out after this thing falling on their head.
So I changed medium and moved to 4" PVC pipe end caps. The bonus of these is they are very light and easy to modify. The caps as manufactured had two negatives
1. Too tall
2. manufacturers printing casted into the lid
So the printing, I used an acrylic burr from my dental armamentarium to grind off the printing, then various sandpapers to make it smooth. As for the change in height...nothing a jig won't handle.
Now that the height has been taken care of, I needed to create a mounting base or flange. So capitalizing on the properties of PVC pipe
After cutting a section of pipe and running a relief cut up the length. I placed it into a pot of boiling water. It softened the pipe so I could lay it flat, but I wasn't properly prepped to create a super flat surface, so I tried again, this time using dry heat (about 210F in the oven)
My lovely wife was not super impressed I was slow baking 'plastic' in the oven, thankfully my daughter had her distracted enough with a hair cut that I was able to set up and execute the job!
Once thoroughly heated I pressed them between 3/4" MDF board.
Then after a night of cooling I was able to cut my flange on the band saw!