Hughes,
I have never owned a dirt bike that I did not fill that gap with duct tape. Keeps quite a bit of mud out of the airbox. I normally tape the frame tubes to the airbox and rear fender to seal the gaps and keep the mud from the rear tire out of the airbox. As a rule of thumb, with the seat off you shouldn't be able to see the rear tire through any gaps between the frame and sidepanels. Can post a pic if you need it. Hope this helps. Cam.