It's always a bit of a problem, especially as a green house is set up to be level, so the gutter has no fall!
I've been looking at this recently, as the small gutter ends you can still get (somewhere), are pretty feeble!
If you have a fret saw, or even a hacksaw, and a piece of wood around half an inch thick, how's about making a template with some cardboard, to the shape of the inside of the end, cutting the wood to the template and getting some clear bathroom mastic to 'glue' it in place? (You'll only need a small tube hopefully).
At the other end, it should be OK to wire up or wedge in place, a piece of ordinary PVC gutter, and point it in the right direction, to collect from both sides and into a butt.
Hope this helps!