Rain is gonna get in what ever you do if driving in bad weather.
I'd go front bar vent...
1. Build a rain trap in the induction pipe.
Straight pipe tee'd or Y'd off to the air box,
dead end the straight bit of pipe about 6 inches awat from the T or Y
Water is heavier so wil carry straight on the easiest path.....i(e) the dead end.
It will hit the dead end,so put a small hiole in the dead end at the bottom edge to drain water away, via a drain pipe/ tube or just leave the hole to draining away to outside the body obviously
job done....
...some water bourne air will still carry on in to the airbox, cant be helped...bit like cheap water injection...LOL
2 or just plug the air inlet pipe in bad weather, and take the lid off your air box.....your not going to be road racing in really bad weather anyway.