Seeing something like this http://www.directindustry.com/comm/l...mper_6214.html
on anothers manufacturers web site I thought that it must be possible to home brew one. Searching around the house for something with the required viscoelastic properties i found two foam discs fron a cdr spindle. They compress and are springy but bounce back slowly. Stuck both of them together and to a 1 1/2 dia 1/4 inch steel disc. Glued the other side of the foam to a plastic gear which was a press fit onto the rear motor shaft.
I wasn't really expecting too much ( or anything really ) but it works! Mid band resonance is signifigantly suppressed and I can get a 30% faster rapid.
It could possibly just be a enhanced fly wheel effect but i don't think so. The inertia of the disc wouldn't be a large fraction of the rotors inertia.
For a first quick hack this shows promise. The best foam to use would be one designed for vibration absorbtion like sorbothane. I found that McMaster Carr stocks small 4 inch squares for about 5 dollars. You can bet the commercial product costs a bit more than that.