We have also considered such a model. But in the end we have decided to take a standard 2 ton Saris trailer with a top. The base is the standard 50 (or 60) cm high and the top adds another 20cm.
We are very satisfied with this solution. It can be locked, it is waterproof under most conditions (except driving in heavy rain, in that case a little water enters from the bottom). The best part is that it is very versatile. The top can be taken off and then it becomes a normal open trailer where you can transport bigger things than it's base surface.
Something like
this