I pulled out the old squirrel trap. It's looking a bit antique (and so am I!) but it works fine.
Time to trap a few squirrels to use as incentive at the end of the go-to-ground set up. The squirrels are very safe inside a wooden box with a mesh front, and are further separated from the dogs by thick wooden dowls. At the end of every training the squirrel is released into the back yard, perhaps a little wiser about traps and dogs.