[Mr Gazza]

I'd thought about the shrinkage, but I thought he said 2%? Either way it's about a mil all round on a valve cover.
I thought about gluing a 1.5mm ply "gasket" on the gasket face and leaving that about a mil or whatever, oversized on the outline, then either adding filler carefully round the periphery or gluing a strip of thin plastic of some sort round the edge.. Maybe even roll some plasticine out to a thickness and add that? It's only the overall of the gasket face that would matter I think.
The inside could be carved out with a burr if required for clearance or at least to save some weight.

Your models are looking better and hats off to your CAD skills. Hand filing the rounds on the fins would save a lot of both cadding and machining. Agreed there are plenty of UK based CNC outfits.

Equally as easy to add mdf fins to an exhaust ring for a pattern? Shrinkage probably not an issue on those?

Yet another way to do this is to make a physical model with oem parts with bits stuck on, then get it "cloud plotted". Some of the CNC outfits have that capability.