I've never done a trunk wall but if I was to do such a thing I would use a resin reinforced double thick panel sized as big as possible to fill the void then I'd use the new Loctite spray foam that is more dense and less porous. Trim down the excess and tape the seams with the thick aluminum foil tape. Now back to that panel, cut a hole big enough for your box to fit through and the box itself would have a 1-1/2 inch flange all the way around from an oversized baffle. This would secure to the trunk wall panel via bolts and t nuts and seal with closed cell foam weatherstripping.
That gives you a permanent wall with the ability to remove the box.
And welcome back btw.