Good one for your first go Dave.
The balance of this kind of piece is, in my experience, often made or ruined at the very outset. As you probably found there is a bit of crystal-ball gazing required as you mount the odd shaped log for the first time. Did you start between centres so that each could be moved, independently, to bring the rim into some sort of balance before a more permanent mounting was decided on?
I ruined many an interesting log before realising my rookie error!
All the best
Brian