I'm already looking at an upgrade, a shaft made to match staton or bikeberry rollers, and a steel spline roller he can order replacements for.
Anyone know if Staton will cut a roller with a 12mm thru-hole, drilled for M6 set screw?
My other option is to couple a standard 5/8" shaft or stubby roller to a male M8 thread, and a bearing mount on other side. I think that's more custom machining, but cheaper roller replacements.