As suggested, I replaced the steel bar handles from the Shop Fox bench screws with cast-iron handwheels from Grizzly. I’m pretty happy with them, though they don’t offer the same kind of leverage as a bar.
Essentially, I just drilled a hole in the end of the screw, tapped it for a 3/8”x16 bolt, bolted the wheel on, drilled a transverse hole right where the wheel and screw met, then drove in a spring pin. Solid!
The only problem is that they stick out pretty far. If I bump into them too often, I might cut the hub of the screw down past the now-unused holes for the bar handles and drill and tap again.
Here’s the wagon vise:
And the leg vise:
On the back of the leg vise I (and my helper) upgraded the casters: