I don't like using my monopod with my long lens (Canon 70-200) because of the balance issue. I started just resting my left hand on the top of the monopod while it held the lens, which was better although a bit awkward.
I thought what I really need is a rest for the lens, so I built one. These things may already exist but I like building stuff anyway.
I figured a small cradle would work for me. I had some old copper grounding rectangular stock, so I hammered it into an arc and cut it off. I then soldered a smaller piece of the arc in the middle for the hole.
I drill and tapped a 1/4-20 thread in it, to attach to the monopod, cleaned it up and glued a piece of leather on.
Works better for me, much less shake, and it's easier to carry the monopod separately without having to take the camera off.