I got this! I've experimented with several different kinds of various materials including improvised... I've used rubber bands, key rings, vibrating etc.
I can tell you the one that will keep you hard and will help you last the longest will be one that goes around everything. Meaning it goes completely around your balls and your dick.
If you go to wish.com they sell some that are magnetic and easy to put on and take off. The one I used at first was a solid metal piece and I would have to wait a while to go limp before I was able to recover it.
Sometimes I like to wear it around throughout the day and it keeps your dick semi big.... Hard to explain and it just feels good....
Hope that helps!