First get the following tools:
1. Hand detail saw with fine tooth blade.
2. Dremel moto tool.
3. Tape off and out line of your opening. See the picture below (pink) outline.
4. Now cut the straight sides and top. Do not go all the way. Just cut till your corners start. If you do this correctly you won't have to use a straight file to clean them up. To make the top cut you will have to use your mototool to cut a slot first.
5. Now using your mototool cut 2 slots (indicated by green lines). Be very careful on the corners, all you have to do is cut a slot and finish with the detail saw.
6. Next with your mototool sand the corners till round. Smooth with a file and some sand paper and you're done.