I didn't like the fit and finish on other raspberry pi NES style cases, so I made my own! It took several iterations before I was happy with it, and I think it's good enough to share now! While I didn't use anyone else's designs for the case and made it from scratch in autodesk inventor 2016, even though it looks like some others on here.
It uses a peg system to align the top and bottom halves and to keep the Pi in place. There are no clips used and you don't need screws to keep it together. It just uses the tightness of the dimensions to stay together but can easily be pried apart with a small screwdriver to get to the Pi board. That being said, depending on how dimensionally accurate your printer is, you may have to sand down the pegs, enlarge the holes, etc. to get it to fit.
I printed mine in PLA, 200*c with 0.2 mm layers. Some sanding or opening holes may be required to get the halves to fit. Many people that have built this recommend to increase the lid length by 2-3%. This was not needed for me to print it, but it may help for others.