I am involved with a retreat center near Cranbrook, BC (which is an hour from the U.S. border near Idaho/Montana), which is looking to heat some 12' x 12 ' retreat cabins in the winter, where it can be -20C/ subzero F in the winter. The current mini-stoves need reloading every hour and a half at night which isn't practical for rentals.
I'd love to learn how to build rocket mass heaters myself, but not there yet - would need a lot of experimentation and guidance, although I love some of the designs I've seen here such as the Cyclone and others.
A challenging part is to know what size is practical for this space. The cabins are reasonably well insulated with windows that are perhaps 4 ft x ft
These are in small spaces but still have thermal mass to retain that heat over a longer time frame, and might fit the bill.
I'm not quite a lumberjack, but that's OK, I sleep all night and I dream all day; I'll coppice trees, I'll grow my food, and compost poo and pee! With a well and off-grid solar, it's a permies life for me! https://www.youtube.com/watch?v=FshU58nI0Ts
My honeysuckle is blooming this year! Now to fertilize this tiny ad: