no need for air tight, your home is not air tight and still works. the easiest and the best are different answers. if you can watch a google diy on how to "frame up" an area, then do that. it is not hard, im a granny and can do it. then just go cheap and use insulated wall board (about $25 a sheet at box store near you). frame for a door, btw! or you can use prefab walls. if you build out a room, you could conceivably run an hvac vent in and use a simple smart vent to add more control to that room. for framing just 2x4s, then the wall board and door, you could get fancy and put up sheet rock over it, but not needed. and since no one is in the attic, you dont need the outter walls to be walls. pick the right place and you are only talking 2 more walls, or at most 3 (talking about building, you have to insulate all).