Insulated
I figured that getting the vent cover off probably helped more than anything, but taking the cover off had been a pain and I never wanted to do it again so I decided to put some insulation in now just in case. I got some flexible insulation from Lowes, the kind that looks like bubble wrap covered with Mylar. Since there were already bolts studs sticking into the airbox I just bought some nuts that fit the thread, pushed the insulation over the stud, and put the nut on the end to hold the insulation back.