You gotta realize that the only difference (aside from water)between the stock top opening to the filter and the bottom that you drill is that the bottom can allow large particles like sand in. But that could only cause one problem......clogging of the filter ! because the particles that the bottom can allow in that won't come in thru the stock opening at the top are too big to get thru the filter anyway. So the only real concern, again aside from water, is clogging of the filter. And even that has not proved to be a problem for me. i started this thread a while ago and i did this mod already. So i don't see a problem even without adding a bottom screen of any type. Of course i said "aside from water" because that IS a concern if you ride in the rain. I don't so to me it's a non issue. But as far as other contaminents getting in thru the bottom, your only concern really is how often you have to clean the filter. And as i said that hasn't been an issue for me at all. Particles large enough to get in thru the bottom that can't get in thru the top are not only too large to get thru the filter, but for the most part they'll fall back down when the suction is released when you're at idle or when you stop the bike.
So i wouldn't worry about it. If you are worried about water then i have no answer for you other than not to do it.But for debris IMO it's a non issue.
