Am searching for window boxes like the glossy black pots the garden centres sell, have tried all the usual Homebase, B an Q, Woodies etc and a few garden centres with no joy
I made my own, I got the normal plastic ones and painted them with black gloss paint; could have primed them first for a proper job, but they will last for the summer anyway. If you can get a dark colour plastic it works better, I've done it for pots too.
I haven't seen any window boxes in terracotta or heavy material, they might not be suitable for putting up high due to the weight/ safety issues if they fall down.