I have found that securing the fiberglass of the twin-pipe cutouts directly to the underlying frame is what I needed to do to tighten the underbelly of the door thresholds.

Exhaust cutout riveting.jpg

I found that without a lateral attachment to the frame rails, the fiberglass in the recessed area for the side pipes tended to "belly out" between rear wheel wells and pontoons. Nothing you can see with the side pipes in the foreground, but enough to cause the fiberglass to contact the exhaust mounting bracket, thereby coupling exhaust vibration directly to the body. Really apparent at about 2800 rpm :-)

These are 3/16" x 3/4" SS rivets. One can back them with suitable SS fender washers to "spread the load". 3/4" Grip range is good to haul the fiberglass in if it is bowed.

Next hack are the grommets I am going to use to better secure the side pipes while keeping the coupled vibration down. Will post photos of that design shortly ...