There are so many different ideas of what to use for a dome topper on the boards. I decided to go with a hand-cut styrene 1mm washer plus a 3/8″ axle cap nut that I bought at Lowes for $X. I rub-n-buffed the styrene washer and painted the axle cap blue.
I think it looks ok.
Also got some resin dome bumps from Izzy and painted those as well.
Axle cap nut: $
Resin dome bumps: $