Dont beat yourself up too much over the c.o.c's...I know of a guy that can close the #3,that doesnt have a good supporting grip,which in my opinion is more important(I guess it depends on what your goals are though)
The things that I normally do for supporting work are
farmers holds,or walks
barbell deadlift hold
db holds,or walks
weighted hanging holds(one or 2 arms...1 arm hurts alot)
sometimes Ill use the rolling thunder,or come up with something else just for variation.I usually do 3-6 sets,depending on intensity.Ive noticed that when I do supporting lifts,my grip isnt closed as tight as it can around the bar......recently Ive been working on wrapping my grip around the bar as tight as possible,which I think might help on the last portion of cloing the c.o.c grippers.