I have been doing girth work with my own custom fit cockring. I can sleep with it on as I have it sized perfectly and wake up with a very beefy cock some mornings. Go to sleep after edging, my theory is you are full of come which makes you more prone to nocturnal emissions (wet dreams) which cause raging hardons during the night. I have woken up with it absolutely throbbing with violent PC twitches. For this reason of super engorged erections you should wear it looser than you think you need as it will get bigger during the night than you have ever seen. If you wake up ok then try a bit tighter.
How to make the ring? simple!
Cut a strip of plastic ring from the bottom of a silicon caulking gun tube. I have used ones 1/4" but 3/4" is great as the pressure is spaced out more so there is less pinching pain at high pressure. I have also used a ring strip cut from the top of an aerosol can cap. Make sure it is flexible so it follows the contour of the penis.
Now get a sock (a straight sock with no heel is great). Now fold the sock over the ring and loop it through and over several times. This is the custom fit and also cushions the ring so there is no pinching. Instead of a sock you can cut a 1/2-1" strip of T-shirt or sweatshirt material and loop it in and out of the ring at a slight angle, circling the ring as you go around overlapping itself like a hanging wrap. Then thinner the material of the sock or strip then the finer tuning you can get when making it a perfect fit.
Fit it on and start to get hard, if you think it is too tight get it off immediately and unwrap the sock a bit. For this reason you should start loose and slowly tighten/wrap the ring until it is a perfect fit. If you get hard and think it is too loose, stay hard for a few minutes more and start edging, it may surprise you and could even be too tight.
If it was too tight it could still be snipped off as the plastic is thin and it is not right against the skin. But please be careful and dont have to do that.
I also like to get hard and wrap with a rice sock and masturbate by moving the rice sock up and down, you get very good expansion this way