For added mass I used (search ebay) "25 Pcs Super Strong N35 Neodymium 6 x 2 mm Disc Magnets " and a steel ring from a $5 Walmart tribet (3 concentric rings, easily pulled apart into individual rings). Then you have multiple rings, appropriate for various sized drivers.

The tribet ring goes on the front, magnets on the back to hold it 'centered'. Magnets allow you to adjust the centering... but beware that if you break the magnetism the little disc magnets will find a home wherever they can in any little crevice (or get stuck to the back of the surround goop). So count how many you start with and make sure you end with the same number. :-) Keep some tweezers or a hemostat handy to pluck them out of tight spaces if need be.