Unless you’re paying around $100 for a practically indestructible bike lock, you’re basically paying for a coated chain and a lock. As Instructables user kill.actus discovered, those are things you can buy and easily assembly on your own for around half the cost.
All you really need is a metal chain, a padlock of some kind, and a fabric tube. The lock just needs to combine both ends of the chain, so there’s not much to do there, but the fabric tubing needs to be sewn together. You can buy some fabric, but an old t-shirt (for example) should work just fine. Just cut out a strip that’s long enough, sew both ends together, and turn it inside out to hide the seam. The tubing should slide right over the lock. You can stitch the ends shut should you want to make sure it doesn’t slide around too much, but once its on you just need to lock both ends of the chain and you’re done. Pretty cheap and pretty simple.