I would do the figure 8, but with a slight twist. Without twisting, overbending or anything extravagant, come back to a walk for just one stride at X and then ask for the other lead. If you need to take few steps to adjust you flexion to the other side, by all means do it. The idea of coming back to the walk is to duplicate an extreme half halt. Coming down to a walk shifts the horse's weight onto his hind end if done properly, so his balance will already be in the right place to take the next canter lead. Once you've got it down to changing leads with just one walk stride, apply a strong half halt in place of it and ask for the other lead as if you were coming out of the walk stride. As you and your horse get more proficient at it, you'll be able to do the flying change with just a basic half halt. good luck.