Cement is available everywhere, it doesn't cost much, is easy to process and absolutely weatherproof. reasons enough, use them to make planters for the garden or terrace yourself.
Finally enough space for flowers on the terrace! If you have been content with a few blooming islands so far, because beautiful terracotta pots are expensive and you don't really like plastic pots, self-made concrete troughs and buckets are an interesting alternative. They can be made in any size and shape, color or paint, decorate with patterns or treat as such, that the surfaces get a natural patina after a short time. The only disadvantage is the considerable weight of the vessels. But even that has its good sides, because they remain immovably in their place even in gusts of wind.
The easiest way to build is with ready-made cement mixtures, which have to be mixed with water and are available for a few euros in every hardware store. Formwork is quickly screwed together from leftover wood.