The cheapest way to cover the bottom is those roll up sleds. You can cut out the hand holds and staple it into the framing. I use an electric carpet stapler so the staples hold good but are not so large as to split the wood.
For the varnish I use the MINIWAX HELMSMAN spar urethane. I like the clear gloss. The more coats you put on, the better it will look. Remember it is only a shanty not a piece of furniture. I also use those foam brushes to coat everything. I buy a package of them and through one away after each coat. The other stuff you mentioned may work, but I like what I use. Remember, this kind of shanty is not going to be left out on the lake extended periods of time. I take mine with me and fish several lakes all season. I never leave it out more than 28 to 30 hours at a time with me in it.
