Help a case mod noob!

buy something like this:
http://www.amazon.com/SilverStone-120mm-Filter-Grill-FF...

Measure the circle inside and cut it out on the side panel with a dremel/drill/saw, drill the holes for the screws, then mount the fan.

Take a look at the links in my sig, I like cutting holes in cases hehe
Another option for you is mark the screw holes using the fan placed (on the inside) on the panel, mark the circle as Yiplong said, then clamp the panel down and drill lots of 3-5mm holes inside the circumference, make sure its all supported underneath AND something on top as well, stops the panel deforming when you attack it  
Moto

Buy something like this
http://www.ncixus.com/products/26519/120MM-ADAPT/Nexus%...

Good thinking Dav   easiest way around the problem imo  
Moto

Then use some of this:
Window molding
http://www.frozencpu.com/products/5096/mol-04/Slim_C-St...
to cover the edges.

http://www.youtube.com/watch?v=7wGlWZ24QsE

http://www.mnpctech.com/casemodstore.html

Follow those instructions and you will do great, I've done something similar for years.

Look what I have done to my case,10-14 C degrees cooler:


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