The radiator in my car has plastic tanks.It also has a tube at the bottom that was used for an oil cooler line that I do not use. I have been capping it off with a thick rubber cap from time to time but I need to cap it and seal it for good. It's only about 5/8" i.d. but finding a similar sized plastic cap is problematic as I don't think a furniture leg cap would work at all. Any suggestions would be appreciated.