This planter/window box is roughly 457mm x 159mm x 140mm (18" x 6 .25" x 5.5")

I printed it in PETG but ASA would probably be the best option. The fence mount holes along the top rail are sized for #6 screws but a #8 will probably work. The base will also fit most stand window brackets which would provide much better support long term than the screw mounting option.

I apologize for the lack of flowers in the photos. We do not plant until mother's day in our climate.