Yes, it can also be a sizing guide in a way. First you start with a smaller diameter and later on you need a bigger one, therefore making sense that growth has happened.
Only cock ring I think of making is the Ring of Power, as it's more utilitarian and All Day wear, if one wants so.
Not very practical for sexy times.
Making a cockring could mean making a mould and then casting it. One could mill steel or relevant material. But overall I don't favor steel or metal cockrings.
I prefer silicone in most rings, a good quality TPR comes second.
If I were to make my own cockring, I'd make a mould and use silicone for casting. Problem would be likely to find the best ratio of silicone, so that you get certain pliability but enough rigidness that it can hold your erection firmly.
And while silicone is nifty material, it can tear easily in certain mixtures.
World is full of choices. If one has tools, materials and the time (and skills :-D ), there are many possibilities.
I just hack saw and file my rods and put on some silicone tubing, Ring of Power, ta-dah!