Parts in glass fuselages
This comment is not to disagree, but to suggest another option for installing formers and other items in composite fuselages. (I've used the method above with success too.)
One of my favorite adhesives for the interior of a composite fuselage is silicone. (or it's hobby variants, like the Zap "Goo")
After sanding and cleaning the parts, you can encapsulate a firewall or other former in the fuselage with a fillet of silicone, and it not only holds securely, but adds vibration damping as well. The other bonus is that the silicone can be cut away to remove the part later, if needed.... Not so easy with milled glass and epoxy!
Just be sure and keep an eye on the arts to insure alignment, while the silicone begins to cure.