Solutions to cockpit heat issue are sort of elusive, and maybe the particulars from a few that hav solved or improved their situation will help ferret out good fixes. I have has limited success, reported below.
I flew this winter mostly in / around Virginia to Connecticut-- not the coldest December/January weather. My 7A is a tip up and has the one heat muff installation. When the ambient air at altitude was 25 degrees or colder, it was very uncomfortable. Particularly when there was no sunlight into the canopy due to nighttime or late day flight away from the sun.
I tried a flow restrictor at the front air inlet and noticed no change in volume (which was already low) or temperature. So, I removed that and added a stainless steel scrubber pad inside the heat muff, which brought about a barely perceptible improvement.
Later I experimented with opening the airflow into the tail cone by removing the top half of the baggage compartment and an inspection panel at the rudder. The result was, if anything, a slight reduction in cockpit heat, though I don't know if that was because more heat was escaping or more cold air coming in. I closed up those panels after a two flight trial.
I bought, but did not install, a second heat muff from Rick Robbins (recommended by Larry Vetterman),
http://www.robbinswings.com/, who I found to be very knowledgeable on RV heat systems. Rick noted that air temp, not air flow, was the most common heat complaint. I held off on installing the second muff becuase 1/ I think I have a FLOW problem, not a heat problem, and 2/ there must be a more graceful solution given that I live in the South, for crying out loud. Maybe if I lived in Minnesota, just maybe....but not Virginia. As another post said, why should I carry the weight and complexity of another muff, more scat, more clamps, etc., for a season limited problem.
Lastly, I tried sealing the tip-up canopy with home-OWNER grade foam seal applied on the left and right mating surfaces on the canopy surface. This made the most noticeable change of all. Still not adequate, but it is warmer in the plane.
Anyway, spring is here finally and the problem has solved itself.

But, before next winter rolls around, I will probably try 1/ a better cockpit seal, and 2/ reviewing the scat tube routing and the heater box sealing at the condition inspection. Once winter gets here, I am prone to try heated seats or better (electric?) clothing instead of the second heat muff.
Mike