Gutters which use round downspouts are common in commercial buildings and high end residential construction. For those who can't source round downspouts and have to adapt to common rectangular 2"x3" downspouts. Best filament to use are ASA or ABS, any UV resistant plastic meant for outdoor exposure with temp. resistance from freezing winters to sweltering summers. Female round end and Male rectangular end.
I chose to print mine at an angle in order to distribute the contact surface with the downspout across multiple layers, reducing the risk of stress fractions/splits.