I had a '72 VW Bus T4 engine sans gas heater option) that we would take skiing up to PA. I had a Coleman Catalytic propane canister powered heater in the back area. It did a good job but put off a lot of moisture. All was good until a friend put his down-filled parka on it at one of the pee breaks - it stunk burned feathers for a week. Melted the nylon too! Probably not a viable alternative any more - especially in small Speedsters.
There are threads here on re-routing the engine heat to the center tunnel vs the side beams where it is cooled by un-insulated steel pipes. Wonder if that side frame could be somehow insulated to decrease heat loss there? Some have also added boat bilge blower pumps ($30 each) in the heater pipe to increase the airflow. The T4 engines did have a similar blower on most engines. This helps defroster volume too.