While googling the soft top for the spyder and trying to find the best pics of how it's made, attached, and fitted I ran across these for a miata. This is exactly what I was thinking of having my mom help me with. I can't sew, but she made her own wedding dress. I figure this is easier. Might be more difficult given I live a few states away from my family, but maybe when they come visit for Christmas she'll bring her machine again (she sewed my winch strap this past week when they were here )
This won't keep the rain off you completely, but would keep the sun off you on super hot and sunny days, keep you a little dryer in a sudden surprise rain, prevent interior fading if you plan to DD the car and park it outside at the office, or just give you options to mix it up. You'd need to make a mount on the windshield and something for the back.
If you wanted to really get fancy, you can make an insert for behind the driver. With the roll bar being straight across, it might be easier to make this instead of the curved piece on the porsche. You'd still need windows, but it's a start.