I'd go 7'x8', 2x4's for the floor, 2x2's for the roof and studs. Studs 2' on center and 2" foam board insulation. Treat your 2x4's and floor deck with Thompsons water seal. Lay scrap carpet on the floor. Sheet it with luan and paint it a light color. Your local lumber yard will have "cover sheets" of steel you can use for the roof. They're the sacrificial sheets on top of a bunk. They'll have dings but it's a shanty. If someone wants to climb up on the roof to inspect it, kick the ladder out from underneath em. If you go with a hole on the end, put your door midship. That gives you two corners to stuff crap into. Set the door 4-6" off the floor so you don't bank it shut. Use 3" plastic gas pipe for ski's. Wire it for 12v lights inside and one outside above the door. The deep cycle out of your boat will last all winter on a charge. Make a solid hitch to pull it. Nothing's worse than a sailing shanty or one that's chasing you. Put in coat hooks and shelves for your dekes and helmets. Bolt a milk crate to the wall to hold a 20lb propane tank and heat the beast with a Nu-Way stove. Lastly, put a winch on your trailer. A 3500 pound hand crank boat winch or a 12v ATV winch, whatever your budget allows. You have 8 months to build it, make a project out of it. Or do what I do, panic in November and build it in a week.