As one of the community has done it for hisself, we have copied a lookalike plate in the shape of stock tube:
mounted:
so the head is pretty closed and it looks like it have to be so from the factory. There is a further weight gain too with 68g instead of 211g for the tube.
Price: 22,50€+shipping
It is available over the Opel-Speedster-Club. If I could help, let me know.
Kind regards
Don