This is a replica of one of the real R2 dome toppers. It happens to be the one I took photos of when I was around this particular R2 when Don Bies had it at Steve Sansweet's Rancho Obi-Wan for a photo shoot years ago. Of all the toppers I've seen, it's the most interesting since it has a beveled edge instead of just being rounded over.

It's aluminum and made in two pieces, so finishing is easy. It fits into the hole inside Daren's alum dome, and is held with an included 1/4" ss washer and ss 1/4-28 nut.