I'm over 6'7" and I've always found straps that were long enough. Granted I don't sling the guitar below my waist.
Having two put together is one way to go. Another would be to have someone who sews make you one or go to a shoe repair/cobbler or leather worker and have one made. Maybe a kid at school could make one as an art project?
Good luck and no matter what length, make sure the strap stays on the guitar!