P100DHG, I know it may be too late for you now, but did you see my May 28, 2020 post # 986 in Edwardb's Gen 3 Coupe build thread?
I believe that I presented a good solution to this. While my demonstrated solution doesn't have the full foam fill, it's something that I can easily do most any time in the future via aerosol foam filling the 3 cavities thru small entry and exit holes.
Did you consider this?

Bob