Well, there is some stuff on the web, like this
(wfs). Other than that, you can have a friend (or a wall) and throw a medball to the side, imitating a golf swing.
On top of that, I'll say windshield wipers
(wfs) or a progression might be a good idea, as would be T2B, but here you touch the bar outside
the hands. And, of course, side plank holds are a must.