Spool Motor Mounts - Poly or Rubber???

The last set of spool insulators I changed (with a good name brand, moog I think?) failed in two years. They sagged to the point where the engine dropped significantly causing header contact and fitment issues. They were made offshore with inferior rubber. I have since filled the air void in the mounts using a good quality two part rubber type of compound (proseal as used in aviation). Any kind of hardening chemical resistant rubber works fine. This stabilizes the mount but still allows some vibration insulation.