The easiest way would be to cut your gasket to match the jug, then use that for a template for your intake
If the gasket is still new, rub a little chain oil on the jug surface, then snug the gasket down under the intake, that'll give you a good "imprint" of the port on the gasket to go by
If you're porting the jug, that's a different can of worms.
gl