Way back when, a Forum member told me a little trick to keep that "loose fitting" cap over the wiper arm mounting nut more secure. Cut a piece of electrical tape maybe 1/2" long. Take that piece & split it down the middle along the length. Then wrap one of the halves around the lower flange of the wiper arm cap. Tape doesn't have to go completely around the cap; just part of the way. That will create a snug enough fit when you insert the wiper arm cap. Mine have never come off after I did this.

Later,