Rubber mounts, heavy solid core solder wound around the case between layers of good duct tape. Keep adding weight until the mounts settle down with the engine revved up. The lightest mounts I've found are holding A/C condensors and lines on 4 cykinder front wheel drive cars in the junkyards....pick the ones from non-balance shaft engine models. Watch for the hour glass shaped ones....dont get caught hauling them out of the pick.a.part in the bottom of the toolbox!
When you get it right, get a piece of the BIG heat shrink tube from the local electric guys to keep everything neat and anchored.
Youll know you got it right when you have to carry spares because they are delaminating from time to time. Have one mount with a thru bolt, loose, through a rubber donut, so the tach cant get completely loose on a run. I have to run one side of a tank different from the other.....similar problem when fuel load gets down on return runs. No more cracks since getting the mounts right, but didnt add mass.
Tune it by feeling the part with your fingertips, trust your feel (use the force, Luke!)