Another option is to remove the axle and use a metal bar across the notches.
Both prongs snapped off my tool when I tried to remove a Regina freewheel so I used the following method.
1.Remove smallest cog - I clamped the freewheel between workmate jaws and used a chainwhip (alternatively use two chainwhips).
2.Remove axle - I think I inserted a screwdriver in the freewheel side against the nut flat to stop it turning whilst unscrewing the non-drive side nuts.
3.Clamp the chainwhip tool in a vice (mine has a metal plate section that is just the right size to fit the notches. substitute with any suitable metal bar).
4.Rest the wheel with freewheel face down onto the chainwhip with notches engaged with bar section.
5.Turn the whole wheel to unscrew the freewheel. This gives more leverage than trying to hold the wheel steady and turning the tool.