i'm with scott on this one,, I don't see what the issue is. I shoot all the time with either a 300/4 and 1.7x af converter or with a sigma 70-200/2.8 and 2x TC, with no battery grip, and have no issue with "balance"
we need a better explanation of how you are holding the lens and camera before going further.
If you want an example of hand holding, here is a shot from the K7 hand holdiing the 300/4 and 1.7x converter (510mm equivalent) at 1/40th of a second.
Just to stress, this shot is 100% about technique. if I can do this, anyone can, the lens and TC combo is in the order of 1200grams plus the weight of the K7. no battery grip no monopod, no extra bits.
and 100% crop of the head