I have used urethane sold as "chimney seal" from Home Depot.
Caulking gun required.

It took awhile to dry. I had made a cardboard dam on the far side of the mount to eliminate some of the mess. ...and another for the near side
after filling. It stayed hard for a long time once it dried.

Best combination.....
I think the best combination would be to get the inserts and
eliminate any air spaces by urethaning them in. There is some
drying time involved. Solid, but less vibration......

Worst one.....
The worst system I tried was to make solid metal mounts.....
too much vibration and noise...I didn't want to hurt the car.....
Removed them the next day......-Bob