[David Dunlop]

I wouldn't think a rubber gasket would have to be very thick for those transmission cover assemblies, Colin. Do any of the surviving pieces of hardware give any clues as to thickness? Two or three millimetres would probably work just fine, if nothing useful shows up in any of the reference literature.

Looks like a seal around the perimeter bolt holes on the inner piece you have replicated, then perhaps another seal on whatever exterior cover plate was fitted, to snug up against the framed rim which faces to the outside of the hull. Not sure what will hold any cover in place at this point, however.

David