To be honest over here it is not done as well, now with the 4min qualify it is something to look into.

I did look into the EFRA rulebook, it only says the maximum carburator diameter is 9mm, it never says where meassured So to me it looks like the bottom part close to the crankshaft must be 9mm and from there you can create the venturi shape all up to the top with the slide in the middle .

But you can taper the hole above the slide a bit that it will fall better in line with a 9.5mm ventururi