Eddie ..... have you looked at the thin metalised 'bubblewrap' type insulation they use for camper vans ...only a few mm thick and flexible .... might this go on your door ??


http://vi.raptor.ebaydesc.com/ws/eB...ds=0&t=1585749527821&cspheader=1

This sort of stuff ..... plenty of sources out there.... link just a random one to the product not a recommendation smile

K