Best and easiest: Hole saw (I use a regular drill).
Second: Jigsaw with a 32 tpi blade (could be very difficult to make tight radius cuts though).
Third: Dremmel with fiberglass cutoff wheels.
Can't even imagine: Torch.
Basically it depends on what tools you have available to you, and if you're willing to buy the tools you need. It can get very expensive if you have to buy everything, however, if you'll be modding alot, then it'd definitely be worth it.