If the vessel is open then the ice is not sterile. You can't use it to chill the ice to pitching temp and still rely on the hot wort to sterilize it.Here in Pakistan we use open small vessels to freeze ice in simmers. Just dump in the whole vessel inside the hot wort and later remove that vessel. But the ice and that vessel should be sterilized from inside out.
Now that I should have thought of.Freeze in plastic tubs, such as cottage cheese containers, having a wide opening and conical shape.