I also don't understand how you can successfuly spray baste if you are going to do the envelope technique. I would think it would be very difficult to get it smooth when you turn it inside out. Also with something that narrow, pinning should be sufficient to hold it together when quilting. I believe you have a problem there.
And you say you don't want to put a binding on it because containers of liquid might tip over. Won't the same thing happen with the edges anyway since there will be a bump on the edges.