No I have never water jet cut anything. I'd love to have the opportunity but I don't know anyone with that kind of machinery

One thought - it may be a good idea to make the base of the rooftop water tank not level towards whatever end you place the tubing barbs since the tank is so shallow and wide. But, I suppose the dynamics of it all depend on how you actually place the tubes in and out. Didn't mean to throw any wrenches into the plan.