I made up a set of aluminium brackets, drilled them all out and alondined Pic1. Next up was mounting them in position with flox. I hot glued a couple of bits of foam to the sides so that the brackets would sit flush with the outside wall. This way the box should mount without air gaps. Pic 2

Pic 3 was after the brackets had cured the next day. I used light clamps to hold the sides in position and started 5 mt gluing pieces of foam. These are the 1 ply BID pieces I’d made up in a big sheet initially. The pic shows the first cross piece in place.

Nut plates

Floxed in place

Box built in position