for some reason i stopped getting emails when people reply. I like the idea of making something to attach to the bottom and insulate - how do i know if my window can support it? it held me - but for a few minutes - i think a few 2x4's, insulation - etc will be heavy long term.
if you could provide a bit more of a layout on how to construct it that would be most helpful - you are in WI so you know just how cold it is these days. I don't know how much room I have for screws into the seat (from the outside up into) just because it's all covered - would you recommend removing that exterior white finish?