How to implement FIND method of EF in Unit Test?

I have a Web API 2.0 project that I am unit testing. My controllers have a Unit of Work. The Unit of Work contains numerous Repositories for various DbSets. I have a Unity container in the Web API and I am using Moq in the test project. Within the various repositories, I use the Find method of Entity Framework to locate an entity based on it's key. Additionally, I am using Entity Framework 6.0.

Here is an very general example of the Unit of Work:

public class UnitOfWork
{
    private IUnityContainer _container;
    public IUnityContainer Container
    {
        get
        {
            return _container ?? UnityConfig.GetConfiguredContainer();
        }
    }

    private ApplicationDbContext _context;    
    public ApplicationDbContext Context
    {
        get { _context ?? Container.Resolve<ApplicationDbContext>();  }
    }

    private GenericRepository<ExampleModel> _exampleModelRepository;
    public GenericRepository<ExampleModel> ExampleModelRepository
    {
        get { _exampleModelRepository ?? 
            Container.Resolve<GenericRepository<ExampleModel>>(); }
    }

    //Numerous other repositories and some additional methods for saving
}

The problem I am running into is that I use the Find method for some of my LINQ queries in the repositories. Based on this article, MSDN: Testing with your own test doubles (EF6 onwards), I have to create a TestDbSet<ExampleModel> to test the Find method. I was thinking about customizing the code to something like this:

namespace TestingDemo 
{ 
    class TestDbSet : TestDbSet<TEntity> 
    { 
        public override TEntity Find(params object[] keyValues) 
        { 
            var id = (string)keyValues.Single(); 
            return this.SingleOrDefault(b => b.Id == id); 
        } 
    } 
}

I figured I would have to customize my code so that TEntity is a type of some base class that has an Id property. That's my theory, but I'm not sure this is the best way to handle this.

So I have two questions. Is the approach listed above valid? If not, what would be a better approach for overriding the Find method in the DbSet with the SingleOrDefault method? Also, this approach only really works if their is only one primary key. What if my model has a compound key of different types? I would assume I would have to handle those individually. Okay, that was three questions?