This is all good advice, but I'm going to argue against: I have a 70-200 f/4L IS, and just got done with a photography trip in the Rockies. My tripod (manfrotto 190xprob + 488rc2) went on every hike with me, and more than once I used that lens on-tripod without a ring to do long-exposure shots of waterfalls and cascades. At no point did I feel I was putting my camera or lens at risk with that combination, in both H and V camera orientations. The balance never felt precarious. It just isn't that heavy of a lens, really.
Would a ring remove any doubt? sure. And I may eventually pick one up. But it sounds like we're in the same situation where this (so far) is just a hobby, and we need to balance purchasess against other expenses in life (like, say, the mortgage
)
Put the money you'd spend on the ring toward a good set of filters for the 70-200 and enjoy it. It was my first L too, and still wows me with its clarity and speed. An awesome investment, and it'll make you never want consumer-class glass again. 