I posted a thread recently looking for a source for a cap for my old aluminum rod tube, but nobody could find one, so was casting about for a solution when my eye fell on the small mayonnaise jar on the counter - lo and behold, the lid would screw onto the tube. So I just put a hold in the lid and ran some string thru it, attached it with Gorilla tape to the side of the tube, and screwed it on. Might work for you, if you have a tube with no lid.