I have an inline tube with an 18mm joint on it. What is the best way for me to plug it up so I can fill it with simple green and let it soak overnight? I've tried the paper towel route, but it eventually gets so saturated that it seeps through the paper towel. Is my only alternative getting rubber stoppers or corks? If so where can I get my hands on some without having to order them online. Oh and what size stopper would I need to purchase. Thanks in advance.