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.
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