Go Pro Hero3 Case with 1/4-20 Mounting Thread.

It's a very tight fit, the tab on the side is loose to allow easy mounting and dismounting. If you want looser fit you might want to scale it about 1% or 2%. No supports required. The only support needed is already in the design, it's the side square window, just brake it off. Let me know if you need a different mounting thread size.