I do a thing when I’m crocheting, where I make a loop by wrapping the cord around the crochet hook a few times, and then crochet over it. Maybe you could make a ring of the proper size by looping some cord, probably shoe lace sized cord, and doing half hitches over it. It sounds like a lot of fun, for a rope person!

I’ve always made a thong style harness to hold toys in

I haven’t found the thong style rope harness for toys to be too constricting in the penis area, however I’m usually doing such things with rope lovers.