In the end I drew up my original gasket on CAD and approached a manufacturer of automotive gaskets to have some of these graphite gaskets made for my car.
These gaskets are very similar to the ones offered in the US (where they seem to be quite popular).
They are 3mm thick but crush to approx 50% when fitted which creates a really nice seal, they are also able to withstand pressure of 140 bar so unlikely to fail during use! They are not reusable gaskets so are likely to need replacing after each exhaust/exchanger removal.
I have run them on my car for the last two years and they have been faultless.
They will fit all 912 models from 65 through 69 and all 356 models that share the same head port as 912

For comparison graphite version on left, vs standard on right

Originally I had a very small batch made but I am happy to consider having an additional batch made if there is interest from other owners.
For ddkers I am able to offer a set of 4 gaskets inc U.K. postage for £30
Please register interest on this thread or pm me directly.

