[Cob] rain catching on a curve

[alex mackenzie]

We have an variably curved flat shed-style roof (one slope only, egg
shaped-ish, but inconsistent) on our cob house that we want to add some *rain
gutter* to. It is a metal roof. We were thinking to use some kind of
flexible UV-friendly pipe slit down the inside curve (it comes in a roll)
and clamping it on like a hollow pacman chomping down on the roof, but a
friend was concerned about there not being enough space for the water, that
it will overflow etc. We were thinking 3 inch?
It would be either attached as illustrated with an L bracket inside every
couple of feet (pre-mounted on the roof, then hooking the pipe over this
and screwing it on) or with under mounts of some kind like a hand holding
it up (in wood, off the protruding rafters). It can't really go anywhere as
it is hugging the roof all the way around.
I like the pipe idea because we can run it around the entire 90 ft
circumference and up top it will prevent under-roof drip runs while below
it will catch water down to the very centre bottom where we would run it to
a cistern and the whole thing is consistent throughout.
Some have suggested custom wood, copper, hand-hewn metal, etc - but this
all sounds either too complicated or expensive.
Does anyone have any thoughts, experience or suggestions before we take the
dive? I would love to see or hear about other examples of this or other
solutions.
We need to collect the water as we are off grid and a well is not an option.
Thanks!
Alex