I will say that I have used this stuff (at work, not on my Vanagon):  

https://www.amazon.com/Castaldo%C2%AE-Quick-SilTM-Silicone-Mold-Rubber/dp/B00LZKA75W/ref=sr_1_1/141-7256671-1564925?ie=UTF8&qid=1522204059&sr=8-1&keywords=castaldo+quick-sil
This is a 2 part "putty" that cures pretty fast; silicone, so can't glue to it if needed.  Nice thing is that you can use a little of it at a time, depending on your application.  But you can smear it into all kinds of things and around blind corners and such.  Think of it as RTV that doesn't depend on surface moisture-cure.  Meaning, you can make it as thick as you need.  Very tough stuff, yet perfectly flexible.  Not positive, but I think pretty good too with higher temperatures.  Quite expensive though.
Maybe just another thing to have in your bag of tricks to fix things.  Or it may give you some ideas.
Smaller (less expensive) quantities are also available should you want to do a little experimenting.

Rich

      From: Richard Koerner <rjkinpb@SBCGLOBAL.NET>
 To: vanagon@GERRY.VANAGON.COM 
 Sent: Tuesday, March 27, 2018 7:07 PM
 Subject: Re: experience with Rubber Molded for engine insulation
   
Hey, I've never used it or heard of it....but MAN, it sure gives me ideas on how to fix stuff, whether on Vanagon or around the house.  Looks pretty robust.  Thanks!
RichSan Diego

      From: Christophe Guilbert <cguilbert@PICASSO.UCSF.EDU>
 To: vanagon@GERRY.VANAGON.COM 
 Sent: Tuesday, March 27, 2018 2:18 PM
 Subject: experience with Rubber Molded for engine insulation
  
Hi ,

I have a 2l air-cooled and would like to optimized the thermal isolation
of the my engine from the exhaust heat. Many of the seals doing so does
not exist anymore so I look online how I could build them and find a
molded rubber I can shape in any seal type. See link bellow ,

https://www.youtube.com/watch?v=FltbX_rUc6Y

Do you guys have any experience with them.

Thanks

CHris, vanagon air-cooled 1983