DIY - replace as required (buy and make in bulk - always have a spare!!)
Set safety glasses - lense of choice (light / dark polarized plain etc)... Easier if they have their own hole near end of arm.
Fishing swivel thing-me (buy in packet of 10)
bit of elastic for necklace enough to go around your neck (JIC - loosely!!)
Measure, cut, tie and sew bits together ..
Does the job - don't feel bad when .... loose, leave behind at showers, run over by car, kid fancies them more than you... etc
My expenditure on expensive sunnies plummeted... about 12 years back... since $25-30.00 a pop (quality safety glasses!!)
cheers
AP