... problem is that I can't use any metal or rubber cockrings cause they are way too small
You can get rubber o-rings from an industrial supply house in any size you want, clear up to big enough to wear as a belt! Only problem with them is that a cock-ring sized o-ring is limited in its thickness (cross section). Also, there can be issues of skin reacting to rubber if it's worn 24/7.
For a change of pace I like to use teflon o-rings and can gang them up two or three at a time (as in the attached pic) to make up for their lack of cross-sectional thickness, and Teflon is safe to wear 24/7.
As for metal I have a stainless steel cock ring that is 2-1/4" inside diameter, if you can't get your junk in one that size you probably just need to work on your technique. The cheap steel cock rings (chrome) can also cause skin problems but stainless is safe for most people's skin.
But my favorite of the non-adjustable cock-rings is the silicone one I got from mr-s-leather.com. As it is 100% pure silicone it has enough stretchiness for me to be able to get into a 2" i.d. sized one of them. The pure silicon products are also safe for constant skin contact.