This is what I did to put pressure on the side after pouring/injecting epoxy behind the fiberglass. I did it in 1-2' lifts. Made a large form that I pressed against the side by jamming 2X4's against the side of my workshop in the garage. Had everything prepped so after putting the epoxy in I could quickly put pressure on the side and left it it overnight. I did it over the course of a week.
The last portion I mad an injector using copper tubing, a plastic bottle, and air blow gun with small tip. I'd fill the bottle with epoxy, screw the cap with attached tubing, push it where I wanted the epoxy, and then use the blow gun to push it out under the fiberglass, then quickly put pressure on the form I made. It actually worked better then I expected.
Epoxy can be messy so I covered everything in plastic, caulke or taped all the holes or seams. Worked good except were some epoxy oozed out a hole where thee dryer vent was. Didn't see it until I pulled the form and plastic and by that time the epoxy was fully cured. I ended up sanding it off and repainting the area. Paint looked good, clear coat was visible because I couldn't get it to blend with the old. They do make a product that is suppose help blend but the paint store guy (who seemed to know what he was talking about) said it didn't last.
Overall I think it turned out OK and a lot better then the delam looked like before I started
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi