Is there a reason that tunnel would have to be open at anytime?
I get a lot of road debris up through there when I'm riding, bugs, grass clippings that are blown out onto the road, leaves, water when it's raining or roads are wet.
I have the fender extension in the front. I was thinking of getting some soft thin rubber to extend it down another inch or so, and use that same rubber material to built a permanent shield inside the tunnel.
That's weird, I don't get this debris through there and I ride with an open-face helmet with the visor up. Maybe the windscreen makes a difference? I swapped the stock one for a FireCreek one years ago. Even water through that opening is not a problem for me.
Maybe 1500s don't do it but I would get water up through the tunnel riding in rain, not a lot but some before I put the tunnel filters in. I do think it helps keep the dust down and helps the air filter stay clean longer. I do know the tunnel filters in my old 1800 got real nasty, I was going to salvage them until I pulled them out of the wreck and saw how dirty they were.
i will not ride without them. i think they make a big difference. i don't like all the air coming up thought there. i keep my windshield vent closed almost all the time. its the extra air i don't want. in the summer the air at 70 mph is like a hairdryer!