As I've said before, in other threads, I don't want to post pictures of my cock online, so I will not post any pics of me wearing these. Sorry.
I hope to do a proper, step-by-step tutorial with pictures in a separate thread. I actually posted one for my first successful model in one of the old Yahoo groups years ago. But for now, I will just give you some details about the materials.
The original model was made using silicone air line tubing for aquariums for the main body. The little spring that keeps the front notched for frenulum comfort is just a small piece of wire, shaped appropriately and inserted into the tubing. For the knobs I had originally used cut off Q-tip ends (not pictured), which worked okay, but needed to be replaced frequently. Eventually I replaced those with fuel line plugs for RC airplanes. Discovering those was a tipping point, for several reasons. RC fuel line, it turns out, is much better than aquarium air line. They are both made from silicone, but somehow different. Aquarium line is stiffer from the start, and gets even stiffer with age. The fuel line tubing is softer, and remains softer over time. I've used it for years and never seen it stiffen. The fuel line can also be purchased in several diameters, which is nice, because I found that a somewhat wider outer diameter was more comfortable for me behind the head. Also, the inner diameter was actually smaller, which fit better for the plugs, which are specifically designed to fit the fuel line. You can see in the picture of the first two models that the plugs are kind of popping out somewhat. This is partly because the aquarium line is old and has lost its elasticity. But also because of the larger inner diameter. Sometimes they would pop out during use, which was inconvenient. I believe Kumakkey uses latex tubing, which might be softer and more stretchy, but I have not found any easily available that has the right inner and outer diameters. So I use silicone, which is fine. In fact, it's probably more bio-compatible anyway, and more resistant to degradation from oils, like CLA, which I sometimes use during retraction.
For the elastic string, I initially tried those stretchy strings used for making elastic bracelets and the like. Stretch Magic is one brand. I tried several different diameters and elasticities, but they all cut into my sulcus too much. Then I tried those little, almost invisible rubber bands for hair (pictured, first model). You can get a huge pack of them at many dollar stores. These worked better, because they were softer and most elastic, but they also broke too frequently. And like the Stretch Magic, they were so thin that they still bit into the sulcus somewhat. I imagine that this is probably a problem with genuine Kumakkey rings as well, although I've never tried one. I eventually replaced the string with a much thicker silicone hair tie called "Scunci, No Slip Grip Evolution Gel Elastic Hair Elastic." They come in various colors. A black one is shown in the second pic, a clear one in the third. The elasticity and softness varies slightly with with different colors.
In the latest model, in the third picture, I used the fuel line for the body, and used a fuel line connector to join the ends of the tubing, to form that little loop. Because the tubing forms a loop, I couldn't just stuff the elastic into the ends, so I punched small holes in the tubing, into which I threaded the Scunci elastic. It fits perfectly, but is a bit tricky to get in there. I use a cut off Q-tip shaft as a tool to push it in to the desired position, and also as a wedge to keep it from slipping too much. It still slips a little bit, every now and then. I've considered gluing it in, but I haven't tried that yet.