Stephan,
Thanks for the advice on looking at an autoparts store. I found a PCV grommet that worked perfectly. I also moved the plate that the airbox sits on towards the firewall as far as it would go to hopefully get a better seal. It's not supposed to rain for a while here, but I might wash my truck later this week and I will check and see if it is wet in there again and post the results. Thanks again for the help.