I use cotton rope. Around 8mm in diameter with a diamond braid (not just spiraling around, interwoven. I'm not much of a rope expert

). They feel real nice when working with them. They do, however, fray quite easily unless you fix the ends with glue or *insert preffered method here*. I'd definetally recommend rope of this type though.
I'm probably going to try for some custom ones now that I've gotten a feel for what length fits me. Sorry I can't be more specific and I hope this post is comprehendable, I'm quite tired at the moment

.