Have you seen the acrylic tubes by Barrow? They're 1mm OD (roughly the same as the fittings) and 10mm ID. Female G1/4 threaded at each end and available in a variety of sizes. Might fit nicely between your two graphics cards. Can't post you a link but I'm sure you can find them on the bay. I'm using sone on my project to go through the side panel from the rad into the case if bad pictures are of help