Ah, a bit like a super version of some packaging foam that sticks little bits of white stuff all over dark clothing. If I go with this method - and it's looking the least intrusive - I'll probably use thin ply and make a double-wall box, so the Celotex is fully enclosed.
I did that with 6mm mdf, Rockwool slab, and a dustbin liner. I used hot melt glue to join the pieces, and stick them to the flap. Not that structurally sound without fixings, but it's enough.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.