Reezvan,
The Jaguar MK2 comes with spats on the rear wheels and I recommend you take a look at that design as one to emulate for your beetle gravel guards,
the spate line up perfectly with the wheel well and are supported by a lip which is bolted to the inner side of the B pillar to one side while it hinges on to slots in the inner wheel arch on the top and rear, it's hard to explain verbally but look into it, its a good simple design.