The most common way is to override CreateContainer method:
protected virtual IUnityContainer CreateContainer()
var container = new UnityContainer();
Your code has one mistake. You create new Unity container within a ComposeMef method which is invoked in the ConfigureContainer method. This is effortless since when the ConfigureContainer method is launched, the container is already created for you. You
can add a return statement for ComposeMef method and invoke it within CreateContainer method.
Hope this helps.