Thanks for sharing this as a reminder of the potential dangers of oar leashes.
When I use oar leashes I use whatever rope/webbing I have available. Most of my frames have 1/2″ tubular webbing but I’ve used a wide variety of short ropes. I’m not sure what specific plastic buckle will hold most of the time and break when wrapped around a leg.
I personally find oar leashes uncivilized. This is one of the reasons I prefer pins and clips as they don’t require me to tie my oar to the frame.