I cut sheets of plywood to fit onto the floor and screwed them onto the floor frame.
Now on to protecting it from the elements...
Tar is nasty crap indeed, but not much beats it for protecting the underside of the floor. I put on a nice goopy amount with a cheap paintbrush.
I'll be installing 1" foam insulation to the floor after it dries for a bit.
I also mapped out the teardrop sides today, but probably won't be able to work on it again until Monday. - sad face-