I would use a 120mm, you will move more air than 2 80mm fans. (surface area), and it can move slower (read: quieter)
As far as making it, a hole saw for a drill will work and you should get a clean cut, be careful, depending on the case you might have to go through two plates of material which (depending on the type of bit you use) can get tricky (should be fine with a hole saw).
Dont worry about making the edges too smooth, your covering it up anyways. and remember to mark off the screw holes too..
measure twice, you only get to cut once.