Hey,you found it-good stuff

You can brush your filter off a bit before removing it or put a filter skin over it-or one of the old woman's nylons.
You could trying greasing the seal of the filter to improve sealing but I don't-though some do.
Is your drain open at the bottom?My 02 has a baffle designed such that nothing could really get up there.
If sand is getting in it will settle to the bottom-naturally- then "bounce" up and stick on the bottom of the filter when going over bumps/jumps.
Regardless,with the speed of a dirt bike,they don't much care where they get their air from.If you wanted to block off the bottom and vent it elsewhere it wouldn't matter.
My KTM's used to put a lot of dirt to the airbox.When riding in a lot of dusty conditions I'd take a layer of old filter or other light foam and stick it dry over the airbox vents.It helped a lot with minimal air restriction.