It is interesting to be writing these posts with so few photos. We were so busy- with pockets overfull and work aprons bursting full- that the camera remained out of sight.
Anyway- We re-did the framing. And put on the thick plywood floor (with zero photos of that-- my kids were champs with the hammers-- and we snapped a chalk line to mark the joists). We chose thick plywood because my floor joists are a bit further than 16".
Then we found polycarbonate roofing material at Home Depot (aka GLAZING). They also have these little foam pieces that fit into the gaps to seal the air out, as well as roofing screws with washers with gaskets on them. I don't like the look of the foam pieces, but used them instead of their plastic counterpart because they looked cheaper and easier (I later covered them with trim).
I wanted to use the glazing only where the sun would hit, and in as few of cuts as possible. I was under the impression it would shatter with my handy circular saw. Online, I saw that people used a plywood blade (many teeth), or tin snips, or made their line in masking tape then cut through the tape. I am here to tell you it is a load of baloney. The regular circular saw blade works just fine. It is a bit loud, and my eye protection was worth every penny, but I had no shatters and my lines were cut fine.
So... we put plywood siding below the glazing on the front and back, and on the far side (and the close side next to the door). I should have painted the siding before installing it, but we live and learn. We chose our house color (dark brown) after priming it. I pieced together the glazing on both sides, and will apply silicon seal where the pieces meet.
On the door and window, the glazing is on the interior. My husband thinks this is a terrible idea, and is inviting rain problems for the winter. I did it because I don't like the look of the foam from the outside. He is probably right, so stay tuned come winter.