RWO Hook and Springs are made from black nylon.  They are available in a range of sizes.  Two hooks per pack.

Item code 6-R8404. Size 4mm: break load 20kg; opening width: 6mm; attachment hole: 4mm

Item code 6-R8405. Size 6mm: break load 23kg; opening width: 8mm: attachment hole: 6mm

Item code 6-R8406. Size 8mm: break load 36kg: opening width: 9mm; attachment hole: 8mm