Silicone
I've used it on a plane I designed, which had a YS .61LS and pipe, and had no problems. (Fiberglass fuselage, prepped inside with sandpaper, and cleaned with a dab of acetone on a rag)
To be clear, the method of firewall mounting that I used was to encapsulate the firewall on both sides, with a small fillet of silicone, smoothed with the rounded end of a popsicle stick.
That experience in particular is the one that convinced me that silicone-type adhesives were good for this application. I especially like it that if you want to, you can later remove the part, by carefull cutting it with an X-acto knife and peeling it off. I've been a little surprised that others haven't used this in lieu of the (much harder to remove, and much more rigid) epoxy and milled glass.