I thought I'd have a crack at making some proper valve covers. I took an approach similar to Donovan's, which was to use a reduced image of a valve cover gasket as a guide, but instead of layering thin pieces I glued the image to rectangular stock and filed and sanded and filed and sanded around it until I obtained the shape. Then I glued them to slightly larger thin stock to make the flange.
The GMC won't be embossed obviously but a decal.