I already have a Forte Secondo for my violin, so this was my first port-of-call when looking for a viola shoulder rest.
The Forte Secondo for Viola is just as good as it is for Violin.
It is very secure when attached to the viola and the legs are very adjustable. They can be changed in size by ~4cm. I have a particularly long neck, and it means that this is one of few rests that actually fit me.
The metal is flexible enough to be bent to match your shoulder shape, but strong enough to stay in that shape. The legs are also adjustable in how long they come out of the rest so it should fit even the largest viola. It fits my 16.5" viola with no issue and it still has room to go larger.
For a smallest violas, I would get the violin shoulder rest. I put the viola rest on my violin as a test and it was ever so slightlytoo big on its smallest setting.