Engine bay sound insulation?

[Esben Bøll]

Finally done fixing up the truck and especially the engine bay. Unfortunately the old firewall dampener is dead and in the trash. I have not been able to find a new or used.
So what can I use instead? I was considering a Dynamat Hodliner but are a bit worried it will soak up spray water and always be wet.
Don't want the rust to come back!
I have already put quite a bit of sound insulation inside the cab but want to get that extra layer on the firewall.

Any ideas how to make a good sound insulation on the firewall