You could go into the plumbing section at most hardware stores and buy “O” rings, they work just as well, and there is a wide range of sizes. That is what I’ve been using and works great.
I’ve got a clamp, but I find it a pain in the ass to set up sometimes. If you don’t wrap, you can hurt yourself with the hard plastic. (I always chuckle to myself when I see a bin full of clamps at the Home Depot). What I usually like to use is a 3/4 inch wide velcro strap. I put it on as tight as I can while I’m still flacid. Then as I get hard, I notice that even though it’s nice and plump, it’s still kind of spongy. I do squeezes and slow jelqing (both forward and reverse) until I feel like it’s going to burst. Works like a charm, and it’s easy to remove in case of an emergency, like bleeding, which can happen sometimes if I get too carried away. I am always left with a plump sausage. The swelling goes down in a day or two, but over time girth gains become more set.