Activave Fitness Ankle Strap for Cable Machines for Kickbacks, Glute Workouts, Leg Extensions, Curls, and Hip Abductors for Men and Women, Adjustable Neoprene Support

Do the job

I got these to use for banded leg curls. The velcro to hold them around your ankles isn't too strong, but works well. Just had to add some carabiners.

Seems sturdy