A Holesaw, dremel or normal drill bit can all be used,
if you want the large hole, use a holesaw or dremel to cut out the metal,
or another option is to mark the four screwholes out, and drill lots of smaller holes inside the fans circumference,
Uther has a great topfan mod on one of his builds,
and heres my method for four 80mm fans in a roof
it all depends how much time/effort you are wanting to put into this and how 'cool' you want it to look
@uther39 surely that's rather inefficient for cooling?
The bottom fans won't do much due to there location. Depending on how the top fan is setup (push/pull) its potentially forcing how air over the cpu heatsink (heating it more) then into the psu which pushes the hot air out of the back instead of the top.
Then the side fan adds even more confusion into the mix.
The design works very well indeed using the heat rises principle and also by achieving +ve cool air pressure inside the case, so you have 2 x 120mm fans at the bottom of the case drawing cool air in and up over all the components, also the side fan blowing cool air straight onto the GPU, then you have the cpu fan and rear case fan exhausting hot air, with the top fan exhausting any hot air lingering at the top of the case.
The proof is in the pudding, i managed to have a 955 x4 in the rig OCd to 3.8ghz, idling at 22c and only 52c on small fft test after 6hours, ambient room temp of 19c.
Remember the issue on this case is that it has not front intake fans, or space for them so i had to mod the case so it was like a chimney, cool air in at the bottom hot air out of the top, pretty much stole the idea from Silverstone cases.
I have/had a few front intakes on mine Uther,
2x80mm in the front spaces provided, and a 120mm in the optical drive area, but as you said, your convection method is based on basic physics and has a headstart on other methods
AHA, thats because you have not seen the under side yet, here you go, notice ive used aluminium drawer knobs as the feet which gives 4.5 cm clearance from the ground plus the fans are filtered to stop them acting like a vacuum cleaner.