Another way...
To get hot air to the back without going through the spar carrythrough bulkhead is to put a pair of 90-degree elbows in the corners of the front seat ramp. They dump warm air right onto the passenger's feet and legs.
I used 1.25 PVC, so there is a pipe running under the seat ramp with a Tee fitting in it, and an elbow at each end. a 1.5 scat tube comes from the right side gear tower, under the little floor plate on the side (can't remember the formal name for that, but its the mirror image of the piece that covers the fuel selector on the other side) and connects to the Tee. I cut round holes in the triangular seat ramp ribs. The PVC elbows have a narrow wrap of neoprene foam for cushioning.
When the fuselage gets back from the paint shop and I put it all together, I'll post a couple of pictures. I think it is going to be great for the back seater.
Another thing that helps is to make a place for the air to get out. You can't expect to pump much air into the cockpit, hot or cold, unless you make a place for it to go out. I made two small holes in the turtledeck skin, just inside the canopy skirt, that allow air to flow back into the tail cone. Another poster in the archives went so far as to put an exhaust hole in the belly that was connected to these vent holes. I think that once the air makes it into the tail cone, it can find its way out along the elevator hinge line and rudder cable holes. But I may yet also put an exhaust port back there.
The other good thing about putting the vent holes in the back area of the canopy is that it will ( hopefully) scavange off the cold draft that comes in under the canopy skirt back there. Just getting rid of that draft will be a big plus for the passenger, and will allow front-seat heat to flow back aft as well.
Stay tuned. BTW, anyone think of a way to do this with parts that are lighter than PVC plumbing fittings, I'm open to suggestions. I'm thinking about molding some glass parts, but....I'll test out the idea with PVC first.