FYI One of the Joysticks I bought is now at a point where you can barley switch it anyway using the twist method as the glue holding the top on is slipping. (and yes I could possibly fix it by somehow getting the top off and re-gluing it) :-)...
Are you sure the handle part is slipping, and not the locknut on the bottom?
The stock handles are molded directly over the shaft.
There is no way you are going to get them off because the shaft has wide spots under the handle.
It took me about an hour to cut/pry one off of the shaft awhile back, and I can't imagine that one came loose at all.
The balltops are a screw-and-glue affair.
They shouldn't slip, without being able to screw completely off.
The locknut/spring on the bottom provide the tension necessary to turn the switch.
You need the locknut adjusted to the point that it just barely allows the prongs on the switch key to rotate below the plastic piece they connect into to switch modes.
If the locknut is too tight, the switch key won't come out of the plastic piece when you release the handle.
If it's too loose, you won't get enough bite on the switch key to rotate the switch itself--which may feel like the handle is slipping from the top side.