Hey I literally just joined the forum but would like to say that I purposley bought the version of my shack w/o a bench seat for this reason. All i simply do is keep a bag in the car with a very warm sleeping bag and a few quilts. Throw the quilts down as insulators and cusion and crawl in the bag w/ all your gear! never been cold or uncomfortable! For a house like yours… 1) if the seats come out just get a sheet of plywood and 2×4’s to make a removable platform, or 2) redesign the backrests to fold back and put platform over that (same construction tho), or 3) just build the platform to go over the top of normally folded chairs. I have helped make all three and all work very well! PS higher=much MUCH warmer & easier.