The best foaming prevention is a ballanced prop....

I've hard mounted tanks often with no problems. I've also foamed them in place using expanding spray insulation (no vibration isolation to speak of from that stuff). As long as the prop's been ballanced.. no foaming.

What I don't lke about the 1 inch dia hole n the firewall and silicone glueing the tank in place: Its very likely to have fuel seepage past the silicone (ithe silicone ALWAYS releases from the epoxy coated firewall within a couple of dozen flights.) There's a glue called E6000 which does a better job.