Mine is more or less stock. I've been searching for an existing mod but haven't really found one. A good start is probably to plug all the little leakage points around the corners and unused rivet/bolt holes.

The main issue is how close the walls are to the bed, which makes a lot of insulation projects dead in the water. If a radiator reflector foil [0] can be made to fit, it might help quite a bit as well.

Other than that, proper active chamber heating is really where we should be heading. When I have the time I might attempt to replace the left panel with one.

[0] https://www.amazon.co.uk/Radiator-Reflective-Thermal-Heating...