theodas, I think the problem with disposable is that it implies paper. If you try to make a bra out of paper or cardboard, it's not going to have the rigidity you need, so it will flap and damage the car that way. Might not last in rain, either.

The design and tooling fabrication costs are specific to each model, too, so paper or vinyl, the engineering costs have to be amortized; so they won't necessarily be cheap with just cheaper material.

Actually the only car I've ever taken a bra off of (the regular kind, probably vinyl-cloth), it had been on since new and the bra itself had ruined the front paint and trim on the car.

Possibly you mean a bra that would stick with adhesive to the front surfaces of the car, then peel off later? Like a sticky-note? I've tried to find an adhesive like this that would work outdoors for another project and it seems there is no such thing. Either they don't hold, or they become 'permanent'. Still, an idea worth pursing.

Truthfully, I think most people that use a bra never intend to remove it and it's just another exterior accessory like mud flaps or louvers.

3m makes a clear vinyl to work like a car bra, i have it on my motorcycles, and it is great for preventing stone damage, and such. like a car bra, it lasts for years, and is made custom for each make and model, cars, motorcycles, even boats. when it gets worn, or damaged, you just peel it off, i have found it fairly easy to remove once you get a corner up, but they sell a special paint-safe solvent as well to remove it.