The creator of rhop refuses to say what materials he's using to create the patches.
Instead, he sells them for $12, which is very expensive considering that you will probably ruin the first batch of 3, since they are quite hard to cut and fit perfectly.
Personally, I have found a small rubber hose which I have cut in small pieces and it worked well. Even bike tube will work, but you will have issues fixing it to the barrel, you'll have to glue it to your hop-up rubber.
Just think to some rubber/silicon hose you may have in your house or you may buy from a local store.