I have an idea for xB skirts, but it seems like it would be a huge amount of work, and easy to screw up.
Make them out of fiberglass, but for each side of the car, jack it up, take a wheel off, and carve some foam into shape to generate a mold, so they actually conform to the contours of the car. They would end up looking pretty slick, and on the hearselike xB, be fairly appropriate.
Of course, I can say this sitting here, having never worked with fiberglass.
Failing that, make them out of foam and bondo. If you do a good job, no one will know.
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