I think the more appropriate title to this thread would be ‘Off Grid’ rather than Non-electric!
The more airtight as you can make things, the better off you’ll be! Adding additional air layers, like a greenhouse inside of a greenhouse would be a route to attempt too.
Depending on how dedicated you are, an investment in some batteries + a small space heater might be warranted and just lug them up to charge in the morning…
Obviously the next step to avoid lugging would be a small off grid solar system which could be repurposed with fans for ventilation in the summer.
The thermacube would limit time on for the space heater to an extent as well.