Aluminum or copper (less effort to form ends) solid (versus stranded) wire or nail. Squeeze in place to make the ends bigger than the hole on both sides of the wall using channel locks. Both ends need to be bigger than the hole. The hole size can be slightly increased if needed to fit the wire/nail. A nail could just be bent if there is enough tail to hold it in place but no way it could touch the valve. Perfect seal is not necessary.
Don't use epoxy or any hardening liquid that could crumble and let go with age, exposure to atf, and heat cycles.