I used a regular sewing machine. Do not need a zipper with a flip over. I have no windows, yet. When I find the right stuff for windows I will make them. Tried it 2 weeks ago and it was too hot inside, but it was warm outside also.
Yes the smitty sled works great for moving the sled in deep snow. I put pipe runners under the sled itself for moving on ice. Let us all know how the collapsible ski idea works out.
good luck with your project.