These are called velosity stacks. The idea is for the shape of the funnel to get a smooth, lminar flow of air going into the carburetor.
The irony is that A) putting the screen right on top disrupts the flow so much it negates any advantage of the stack and B) it doesn't filter out anything bigger than a medium sized bug so the engine is going to wear out fast. Even bolting a cone type air filter right to the stack isn't so great. The stacks really need to be run open to the air to work properly.
The proper way to do it is build a large air chamber around the stack and place a filter on the chamber. If you really want to go whole hog you can provide a positive pressure to the chamber. Lots of sport bikes have this exact setup. Throttle bodies connect to a large ari box, there are stacks inside the box, there's a filter on top of the box, and the lid to the filter is connected to ducts that face forward on the bike and channel air in to create positive pressure.
Scott