template template parameter

[Caligo]

I have a struct that looks something like this:

struct Box(T, size_t width, size_t height){

alias width width_;
alias height height_;

//do something with a Box of different size
  void Fun( B )(B b){
    // using width and height of b
    B.width_;
    B.height_;
  }
}

auto b1 = Box!(double, 2, 7)();
auto b2 = Box!(double, 3, 4)();
b1.Fun(b2);

I think the technical name for this is template template parameter.
Using the above I'm able to do what I need to do, but is there a
better way?
and should I use alias or enum?