Curves... they suck.
I've gotten quite used to bending plastic sheet in layers (laminating), using bondo, easter eggs, or in the case of the roof here, you guessed it... styrene tube cut in fours length-wise. I used the same thing for the diner corners. A while back I got 6 feet each of 1.5", 1" and .75" styrene tube on clearance and use it a LOT. The pile of bulk at the front (and back) of the car roof is already smothered in Bondo waiting for me to sand it smooth.
For cutting the tubes, First wrap some tape around the tube and make a slice along the length of the tube to use to measure the circumference. Remove the tape and measure it, divide by 4 (or whatever you need). Then cut some strips of tape in that width and place them lengthwise on the tube for guides. Then a strip of Dymo (label) tape works great as a straight-edge. Score the tube along all the tape edges then squeeze the tube and POP! Curved segments abound.
For the curve around the front to the sides, I just used sheet styrene bent around the guides seen in the frame photo. The side-walls are Plastruct corrugated sheeting. I cut it in the shap I needed then bent the front around a marker pen to curve it before gluing it down. The I glued rod around the outside of the corrugated sheet to make the rim. Half-round was used for other edging.
I plan to light the inside and head/tail lights but am waiting on an order of 10mm warm white LEDs from Hong Kong (ebay + HK = cheap electronic components) before I can wire and finish detailing the inside. The headlight is from a tip of some ABS "T" connector from Home Depot. I ofen raid their plumbing department for interesting shapes.