I make a "crutch" when using rock maple motor mounts. I first drill the mounts for the engine and then bolt them to the engine. I then build the cross members and prep it for epoxy to the fuselage sides, usually using internal ply doublers though I have used balsa doublers too. The longer the maple motor mounts, the more vibration dampening.
When I used to build one piece C/L models I would actually glue the end of the motor mount to the wing sheeting when gluing the wing into the fuse by sliding hard balsa shims between them until it was pretty tight. This allowed the vibration to spread out over more structure. Typical model life was 1500 flights or so, most hang in my garage at 20 to 30 years old! It's pretty effective.

So far all of my R/C models except the King Cobra have nylon or aluminum mounts on vertical firewalls.

Chris...