replying to Fevric J. Glandules, Iggy wrote:
Thin (or thick if you like) foam-board covered by aluminum or plastic
sheeting. The upper glass can be sheeted-over with Plexiglas or other clear
plastic, to create a dead-air space and insulate quite well. Both are best
done on the outside, where protrusion from trim isn't noticed. And, new
weather-stripping will also go a very long way.

--
for full context, visit https://www.homeownershub.com/uk-diy...r-1246781-.htm