Appreciate the encouragement ... sometimes it's hard to judge how folks will respond in newsgroups these days ... it's a risk when you're not a regular.
So now I'm like a dog with a bone :). How about we keep the idea of the cladding on all four walls, let the cladding extend across the top to the edge (holding the glazing in place), and let it get enclosed in the supporting structure at the bottom of the box (non-removable). BUT, we leave one upper edge in the angle format (the angle you have now) so that edge could be removed and the glazing slid out (think a poster frame with one edge removed). Add some gasket design work, and you might avoid caulking & adhesives there, too.
I think the plasma or HCA treatments will address that concern pretty well (ref. General Magnaplate, Duralectra). Plus, they have lots of colors ...
... and a final thought to eliminate the penetrations. Keep the cladding full-length as above, but add tabs at the lower edge to bend over the wood frame and fasten the cladding in place (beating the poster frame analogy to death, think of those bend-down tabs that hold the cardboard backing in place). Then put a fastener through the tab to lock it. I think the heat-loss here would be small, and could be further reduced with an insulating layer along the tabbed edge.
Aero