A great way to simulate what it's like to dig random balls off blockers' hands on scramble plays is by using a fit ball. As you'll see in this drill shared by Mark Barnard, head coach at Oregon State and former head coach of the Australian national team, spikes that deflect off a fit ball could very well go anywhere.
This drill helps defenders develop reading skills and teaches them to always be stationary when the hitter makes contact.
Another reason Barnard favors the fit ball as a training tool for digging is that it's an easy target for the coach to hit. And, as he points out, it's even a good shoulder and upper-body workout for the player holding the ball.