I am a paramedic so I have easy access to these, But I used what is called a sam splint http://www.sammedical.com/sam_splint.html
its basically i mold-able piece of metal covered in foam that i cut to fit directly in the center (snugly) and fits the length of the tube so when the end cap is screwed on it touches both ends. I used this to mount my lights so that i could remove them from the tube if i need to.http://www.homebuiltrovs.com/rovforum/viewtopic.php?f=18&t=696
if you do use this method just remember to crease down the center to give it some rigidity.
(im going to be playing with my ROV at the lake over the next week, I will try and get some pictures of what im talking about.