Sorry it took me so long to see this!
and welcome!
It would be real nice if you could do a ground to air heat exchange, but you would need to dig down 4-6 feet to install it.
Concrete. Take a look at these two methods:
https://ceresgs.com/solar-gree...
You could do the Swedish Skirt around the perimeter, or you could dig down, insulate the dirt walls, refill, top with gravel and pour a slab. These would be the two most efficient IMO.
If you can't do anything like that (I couldn't do either), go with crushed rock tamped well, a good layer of paver dust tamped well, and a nice dark color brick or stone that will absorb a lot of heat. You could put rigid insulation somewhere in that sandwich, but I don't know how well it would stand up over time.
BTW...expect water on the ground...a lot! One thing I like about the dark bricks I have is they absorb some water and help evaporate it fast, as opposed to the water having to "find" the drain.
Please let us know what you do. Now, I should go back and read your 2nd post.
