Outing myself here - this is where I cheated - using treated dimensional lumber as floor joists, with hangers. But you'll never see it unless looking up from below. The rough sawn 3/4" pine sub floor, walls, and chinking will hide the evidence. I'll cross the subfloor with more rough sawn planking, with edges beveled with a drawknife, to create a monster-thick, rock-solid floor. More on the floor, where I got the wood, and other stuff soon.
Note -- nothing like a little help from your friends. Here my friend, "Buster" aka Justin, poses with the "persuader" mallet I made.