Hah ! I've gone along the same lines for my latest attempt. Went back to the first plastic enclosure which is a tight fit. Added a big wrap around alum plate internally with an extra pair of fins over the TEC area. Then added a copper contact finger to the tripod mount screw for some minor metal to metal contact. I haven't added an internal fan but that is an option.
Big change was the insulation method. Front area is internal styrofoam but the rest of the box is externally wrapped in multiple layers of bubble plastic, bubbles facing in to create an air cell insulation skin. I've seen\heard of it being used, single layer, to give window insulation when you can't afford double glazing. The back of the box also has an additional layer of styro but overall it has made for a smaller package and very light.
Testing is yet to happen due to other obligations at present. Stay tuned ...
