You might find that a station wagon or van roof may be your best bet. You can make a length-wise and a width-wise template of your roof and take them to wrecking yard and lay them on as many as possible to find the right contours. You might also check the sedans and pick-up??? If you flange you leave a place for moisture to accumulate so butt welding may be the best on a roof, you'll have to be careful with heat buildup and warping.
