I enjoy scrotal/testicular bondage, tying myself with ropes, belts, toy handcuffs, padlocks, Velcro straps, et al. Pulling on the balls gives a pleasurable sensation along the spermatic cords; even some pain is desirable to me.
The pet store has leather and nylon collars with a ring already attached. Available in various widths depending on how much you want to stretch the scrotum. Second hand store has narrow ladies belts 1/2" to 3/4" that work well. After cutting the strap to length and punching/drilling proper sized holes for a tight fit, punch another hole near the end of the strap. I put a small padlock on putting it through the leather and the buckle, locking it in place.
Ropes & plastic tubing: I like soft polysester rope 3/8" I got some oxygen tubing that is nice also, as when knotted, the friction keeps it from slipping. Knots that slide: two half hitches, hangman's loop, etc, while they work, and are quick, long pulling may cinch it up dangerously and crush something. In old nursing texts they advised, for attaching limbs to a bed, to use a clove hitch, and tying an over hand knot. Then one can pull, and it won't dangerously tighten. A more elegant knot--my favorite--is the 'bottle knot.' It takes a few minutes to learn from youtube videos, but worth it. Cinched up around the scrotum the knot has loops on both sides so the balls can be pulled from the front, or back, or both at once.
Handcuffs: Toy handcuffs are cute looking, increasing the bondage feeling. If the balls are too small, and slide out, tighten the moving part until the balls won't escape. Noting where it is, drill a hole in the fixed part where it would intersect the moving part. Now put it back on tight, and mark the other part with a felt tip pen. Drill hole. Now when put into place an lined up, there will be a hole straight through; stick a 12 ga piece of wire and cuff is in place. Bonus. Cut the other cuff off, leaving couple links of the chain. Now attach a new pair of cuffs to the one on the balls---sub his handcuffed to his balls!