Is there not a plastic flap over that lid when you open it up Lee ? Ive got one on mine il take a pic if you dont know which bit i mean , mine seems to collect most of the crap before going in and i very rarely have to clean out my airbox.
There should be a seal around that top lid as well like a rubber tube all the way round which seals it.
And yeah you can get the wider flap from gas gas uk its worth it anyway cos it covers your shock as well so you dont get crap down there.