I really can't answer why Stihl makes that opening so large. I guess so that it will easily, and without fault fit into their proprietary air filter assembly. They are the only ones that make such a large opening on the regulator cover that I know of, but then the others take the chance that cooling air pressure in the chainsaw, weedeater, blower, or whatever, might cause the same problem you are having. Anyway, most all carbs just have a 3/32" hole in the cover. I drill them out, solder in a 1/8" od tube, and from there run a hose into the fuselage. Some guys even run the tube into a balsa box with a pin hole in it, or an old 35mm plastic film canister. I've never found that necessary myself.
AV8TOR