How to mock an abstract class using the decorator pattern and containing a constructor that takes an instance of itself

First, I want to clarify that this question is not a dup of this because the situation is different when you have an abstract class.

My situation is that there exists a base class

public abstract class FooBase
{
     public FooBase(FooBase inner) { /* ... */ }

     public virtual void DoSomething() { /* ... */ }
}

which is in a different assembly and I want to mock the behavior of DoSomething(). The problem is that if I try to create a mock

public class MockFoo : FooBase
{
    public MockFoo(...) : base(/*I have to put a FooBase in here*/) { /* ... */ }
}

I can't pass a new MockFoo() into base because it'll cause an infinite recursion. Any other ideas I have (such as creating another MockFooOuter) can't get around the infinite recursion problem.