Cabin heat in a Sonex can be challenging with the Jabiru exhaust pipes. They are complicated to fit a heat muff to, and that limits options. One solution is electric heated seats.
Amazon sells a really nice universal seat heater kit for automotive use. The kit includes seat bottom and seat back pads for two seats (4 pads total), two hi-off-low switches, control relays, and fused wiring harnesses. The heating pads are a woven carbon-tape mesh, have adhesive backing and can be cut to fit. Each seat draws 3 amps on low, and 5 amps on high. The Jabiru alternator has no problem at cruise RPM with supplying enough power for both seats and the panel instruments, but at low RPM you'll likely need help from the battery. I wait until takeoff to turn the seats on, and this works out well.
Installation is simple, and can be completing a only a couple hours. Open the seat cushions and stick the pads to the foam inside, route the wiring harness to 12V power and ground, drill two holes in the panel for the switches, and tidy up any excess wiring. That's it. Sit back and soak up the warmth!
Universal Seat Heater Kit (Amazon)
SonexBuilders.net Cabin Heat Discussion
NOTE: photos link to full size image