The more common purchased ones have a I.D that's the same as the flared out part of the carb inlet.

The O.D. is small enough to just clear the socket head bolts used to hold the carb on.

Length to suit.

Outer end is cut at a angle generally placed into the wind.

The base and the tube are separate. The base is held onto the carb with the carb hold down bolts. This base retains the tube.

There is a little lip on the carb side of the tube that fits into a counter bore of the mounting flange.

This way, the tube can be swiveled when loose to any position desired but held firmly when the bolts are tight.

You can make the bell mouthed ones according to the above mounting parameters.

Enjoy,

Jim