Robin-- many of us have been down this road, and dare I say back again.
With that heavy lens, if you want to do it justice, you will just have to make the jump to something that will hold it. Like many of us, you will soon tire of ruined photos because of lack of sufficient support. You might get by with a sidekick, but then you will have to get a ball head. A gimble head IS the answer. Tools don't make the job, but they sure can make it easier. I have a GImble head extra, as I just replaced it with a Wimberly II - I am asking $235.00 for it, shipped, with extras. PM me if interested, and it comes with some extra plates and knobs. Like new it is. Just a thought. As I said, I just got tired of rined photos. Bags can help, but they are not the easiest to carry around. I just went over the top and got a TV33 from 'Really The Right Stuff', and every screw and nut is made in the USA - it is carbon fiber, but they have others. Worth the money to say the least. Feel free to PM me anytime.