Hi there,
It's interesting to see how you've done the curved sections of the wheel arches.
Birch plywood is often used here for similar projects to yours and it comes in a huge range of thicknesses. I have a friend who builds some unusual speakers and he does curves like those buy laminating a 5 or 6 layers of 3mm sheets. It is a laborious process but the resulting curves are obviously extremely long. You can also save weight because you can remove some or all of the formers after the layers have dried.
If you have have sections longer than 4ft you have to ensure that the grain is predominantly running along the 8ft axis of the sheet as it makes it much easier to bend without snapping.
Both methods work fine but I thought it worth mentioning anyway.