I plan to fit an Austin Gipsy heater over the winter to my Light 15. It is a rectangular design with flaps at each end and fairly slim so it will not intrude too much. The car runs cool even on the M25 in the summer so a thermostat will be needed, either fitted in the top hose or in a separate housing. The housing from a Rover with a K series engine looks like a possibility.
Cheers Richard
Richard Larter