The nylon collars around the pin on the shoulder harness can be purchased from any cessna parts suppliers.  I believe that I got mine from Aircraft Spruce for under a dollar each a couple of years ago.  They can be stretched over the pin on a nice warm day or you may want to use a little heat from a heat gun or blow dryer.  Definitely do not attempt to remove the pin.

I tried the hot water trick but my washers did not soften enough to slip over the steel retaining stud.  So, I cut the nylon washer, slipped it over the stud, and glued it in place with extra thin crazy glue (ca. glue, purchased from a local hobby shop.  This glue is thin as water and seeps into small cracks.)

I have seen quite a few Cessna owners use a zip tie and a pandit gun.
The zip tie is placed around the shoulder strap retain lug and pulled tight. 
This has worked for years. 
This must be ok because I used to fly Part 135 in a Cessna with this repair.  The FAA did not seem to have any heartburn.  Our inspector was a real stickler for details.