Repair holes in roof?

Salt water dripping from my roof box ate some holes in my roof, and I'd like to repair them. I'd also like to add some soundproofing. I was wondering if it was feasible to get another roof from a salvage yard, trim it to size, and use an adhesive to glue it - from underneath, on the inside of the cab - to the existing roof. Then, I could easily bondo the rust holes from the top. The bondo would be thin, since it would only go down to the "second layer" roof, and would not be subject to much flexing or vibration.

Finally, I would Dynamat the underside of the new roof. I did the rest of the interior years ago, but didn't want to remove the headliner, so I left the roof alone. This would finish that job.

Any thoughts, opinions, or advice on this?

Thanks in advance!