You can use VP-UML, community edition. It is free and it has the Composite Structure Diagram. Only today I used it this way.
And notice, the modelling tools are not nice, look here: https://stackoverflow.com/a/21727625/715269. Notice some numbers that are written vertically. I don't want it, but I can't control it. And EA(not free variant) is less nice and IBM Modeller, that is free, is MUCH less nice.
Diagramming tools can be nice, but they do not support modelling - check for correctness and inter-diagram dependencies. Of them, I found MS Visio acceptable, but it is not free.