Snap ring for canvas-sails - OpenScad

On the ship we often made simple covers from tarpaulin to protect equipment from cargo dust during loading/discharging or against weather condititons. The easiest way is to make few holes in canvas, put a rope through and tight. It looks... well- imagine, and sometimes fabric tears.
So I invented simple ring to snap in a hole.
If not fit, put a little glue between stringers.
As far as I know something similar is used to reinforce sails corners.