Here's the secret to leak proof valve cover gaskets. (
Oil pan gaskets too)
Obviously, the valve cover gasket surface has to be flat. Indents where the bolts go through must first be flattened.
When using stamped steel valve covers, use the little metal ears under the bolts to spread the load out.
On the valve cover side (and it doesn't make any difference if you use rubber or cork), I use hardware store Weldwood/DAP contact adhesive. Spread on valve cover, and gasket top. Let dry until tack free, then join surfaces.
On the bottom of the gasket, I use plain old wheel bearing grease, liberally applied. It seals great, and comes apart for maintenance without destroying the gasket. The gasket is re-usable.
Oil does not dissolve the grease when used as a sealant. Have been using this technique for years on valve covers,
oil pans, and carburetors.