Rainwater Catchment System Question

[stonedragonfarms]

Hello:
I'm looking at installing gutters on my greenhouse to catch rainwater which I will then channel into a pair of 500 gallon cisterns. I've seen a number of demos on instillation of the gutters on quonset structures, but each has been on a structure that is either glazed with rigid poly sheeting or single layer poly (more of a tunnel application than a gh application), but none on instillation on a double sheet inflated poly structure. Do any of my fellow growers have any thoughts on installing gutters, while at the same time maintaining that inflation of the sheeting? I am thinking that I may just need to rig an external (non-frame attached) support structure to use; anybody have any other suggestions? Rafters are on 4' centers, in theory I could attach gutter brackets directly to them, but I'd worry about tearing the poly when installing...any advice?
Thanks!
Adam