Just an update,
I took the top frame off of my 20H and cut a single wier-type slot along the back. I figured that I can always add acrylic teeth or even eggcrate later if I need. It actually went a lot faster than I expected, the whole process took only half an hour. I did it outside with a garden hose running over the surface. After getting a feel for the dremmel I had no concerns about cracking the glass but I think I may have shortened the life of the bit by going that fast. I've just attached the overflow box to the back and am ready to order my lights from DIYreef. Thanks for the excellent advice in this thread. In my opinion this is the best way to put an overflow in a nano!
- Chad