Like some folks already mentioned, you can get filters that go on each air horn. For example here is a Miata with a set of R.S. Aizawa ITBs with such filters:



There are also "cake" air filters available that go around all four air horns.

Cars that come from the factory with ITBs (BMW M, Nissan Skyline GT-R, etc) usually run a plenum which surrounds the airhorns and has a tube going to a filter at a remote location. TWM makes a similar airbox for their ITB kits. Here it is on a VW:



But TWM states that this airbox is not designed to be pressurized, so no dice with a forced induction application.