Your solar furnace and Solar hot water builds are very cool. I watched some of your youtube videos a few days ago as I work out my first off grid build. My brain is completely worn out from trying to piece together a solar array, I feel like it's similar to learning a new language, so the simpler more tangible alternative energy devices are kind of relaxing to think about.
Along the lines of sunshine_eggo, I was wondering about some kind of baffles in the box, not necessarily to direct flow, but more like heatsinks to create more surface area and potentially hold heat longer. Maybe chunks of black painted 2x4 or something to give some extra area, and maybe a little bit of restriction. (I'm typing slow, I see you thought about this in your replies).
On your other build, if I remember correctly (brain fried from solar math), I believe you had the fan blowing from under the house up through to the higher entry into the house. I'm not an expert on this stuff at all, I have some experience with how I currently heat my house with a similar duct system from a wood stove in my garage, but I'm really just experimenting with what works and doesn't. It may not be possible in your situation, but I'd be curious to know what affect switching the fan orientation would have. Example, moving the blower to pull the heat through the box from the top and natural air return from below. Do you know what kind of temps and CFM you get on the output side now and how that is different from what's in the box?
To give you some reference of my current experience, I have been heating my regular house for about 5 years with a small wood stove in my garage. I built a small room around the wood stove (basically my room is your solar box), built a hood system over the woodstove which goes to about 12ft of 8 or 10" duct pipe across the ceiling, to a variable speed (600 CFM max) duct fan that has a thermostat built in, to about 12 more feet of pipe into the house. When the warm air heats the blower enough, it turns on at whatever I have it set at (usually on around 85F). I have a wireless thermometer in the duct and typical temp coming out when keeping a nice burn going is around 100-115F. Blower is supposed to be rated to 140f from my memory. I don't know what the exhaust CFM ends up being as I don't know how to measure other than "feels pretty good". For return air, I just leave the door between the garage and house open. I can keep 1400 SqFt in the high 60's pretty easily all winter as long as I'm home. Temps here usually average in the 20's during worst part of winter I would guess.
Always great to see new ideas since the more options people come up with, the less dependent on grid resources we will be.