You can buy the exact thing you describe from Davis Model Products, a rubber sleeve goes in the firewall and rubber washers go on each side of the firewall. You can tune the damping with tighter and looser applications, also you can adjust the right and down thrust by tightening certain bolts differing amounts. I use them on my Diesel applications so the airframe doesn't sound like a drum. They don't let the engine move much but do an excellent job. You can add more washers to the front for a softer mount.