When I installed the Vetterman exhaust system I was able to add another muff. That helped along with the tube referenced in the linked thread.

I think the main issue with getting more heat is the lack of a true muffler to wrap a muff around. Keeping these planes light is always a compromise.

Doing a better job of sealing the Windows helped more. I need to look at that closer again as my lower door closing rod is getting some wear and that is allowing some air to come in along the upper door window in the front door post area. It is easy to find these leaks when you're crushing at 90 and it is below zero outside.