Fabric convertible top design for a 250 mph+ supercar?

Let's say you were commissioned to develop the convertible soft top for a 250 mph+ supercar.
If you say it can't be done, then you are shown the door and you wasted your once in a lifetime opportunity to get a dream job at a supercar manufacturer.

They need a soft top as it's huge mid-engine leaves no room for a retractable hardtop.
Also the market segment calls for a convertible with power operated roof. No manually folded roofs or take out and stow away targas are allowed.
Also some soft top nostalgia.

Maybe you will need a super-strong fabric like those used in seatbelts or Kevlar?

Maybe the fabric can be knit curved to both be made in one piece and to have its curved shape enhance aerodynamics? Pretty much like those hand knit half-sphere shaped crotchet hats.

Maybe you can allow the custom knitting to get bow attaching points included in one piece.

Maybe you can get a superstrong, (300 mph) wind resistant outer fabric even if it allows some water to leak down, and get a second weather resistant fabric underneath?

How would you design the folding mechanism and supporting bows to withstand 250mph aerodynamic forces?

I think you're missing the main point here, the strength of the fabric is not the main problem.
The airflow around the car isn't flowing at constant speed let alone at a constant direction. A whole range of different turbulences develop all around the car, and the roof is among the most problematic areas because air increases its speed (relatively to the car of course) right after the top of the windshield reducing pressure, and creating lift.
This airflow isn't constant in speed or direction as I said, so you constantly have different waves of pressure hitting the top of the car. The thing about a fabric is that it's not rigid, it will move, basically vibrate under the effect of the waves. This means an incredible noise and possibly physically avertible vibrations too, and mechanical stress on the fabric (which I think it's not the main issue).

Indeed if you look at most high performance cars, the fabric roof is either just a portion (Pagani Zonda Roadster) or very, very tight (Mercedes-Benz SLR).
So it's just a matter of producing a mechanism that, as on the SLR, is stretching the fabric to such an extent that it won't be deformable when hit by the waves of pressure.

Think of your scarf, with an apple in the middle, and you keep each end of the scarf in each of your hands. If the hands are too close, the fabric isn't stretched if not by the apple, and the scarf is completely deformed by the apple. If you can reach the lenght of the scarf moving away your hands from each other, the scarf will be flatter and flatter. The pressure waves are like the apple, just bouncing and moving around all the time.

Knowing the level of quality delivered by Mercedes-Benz in its high end cars, and the minds at McLaren which likely had a say also on this aspect, I suspect the SLR's top is your solution.

The weak point would then be where the fabric is glued or sewn to its frame, which I think is not a big deal.

Electroactive polymers I reckon are the way to achieve it.

"synthetic muscles" could be flexible - possibly enough to "roll" into a tight ball rather than all that floppy material current ones have to move and store.
Once in place, electricaly activate the polymer and bingo how tight would you want it to be ?

Tidy up the loose corners (no pun intended) and it's a winner.
If you can afford a soft top car that does 200mph+ then you can afford the material :)