Bounce handstands off of a yoga ball to their back on a mat, focusing on hitting handstand. And TONS of giants on straps, if you have a strap bar. How are their cast handstands?
ETA: Just remembered a drill I used to have to do. Handstand against the wall, walking hands out as far as possible, back facing the wall. From there, lift the legs over and up to handstand (with a spot, making sure that the back isn't arching). This can also be done on a floor rail, or with lots of mats stacked up behind the low bar. (If you do the drill on the low bar, you can have them do it from a cast handstand. And then you also have a drill for leaning over the bar in cast handstands!)