What I do is this:
1) Bring in the box into a canvas that is a a small bit wider, and about 2/5 taller than what you want to make a reflection of.
2) Duplicate the box
3) Rotate copy 180 degrees.
4) Flip the copy like like this: <-->
5) Bring duplicate down below the actual box.
6) Lower the opacity to around 10-20%
I hope that helps.