Installing the stereo and the heater today. Yup, heater. Found a 300 watt 12V ceramic heater. Did a test, it draws right at 20 amps, so with a 75 amp/hour battery I should get 3 hours out of it, give or take. Not great but will be nice for knocking the chill off, and I suspect the small cabin volume will mean I won’t have to run it for more than maybe 20 minutes at a time, maybe less.
So, cutting big ol’ holes for the speakers and generating mounts for the head unit. Looked hard at the round gauge style radios and went with a single-DIN conventional unit. Mounting isn’t that hard and mucho better features. Doing Polk Audio 6 1/2″ cabin speakers and 4″ galley speakers. I’ll use the front/rear fader controls to route sound inside or outside as needed.