I couldn't get this one in time for the Macmod Challenge, so i will post it here and at the Aqua-Mac forums.
I wanted a blow hole on the side of my computer for some time, but it wasn't until now when i got the parts to do it with.

Tools uses...
Dremel with several cutting discs, Hot Glue Gun, 2 Glue sticks, Clamp, Duct tape, Pencil, Sandpaper, Hammer.

First thing i did was cut the grill from an old scanner power supply that i got from a recently unused HP scanner.

Here is what's left of the power units from the HP scanner.
The next thing i did was trace out the 120mm blow hole with a roll of duct tape and a pencil. Sorry folks, no pic of these, was not thinking about pics at the time. Then from there i used the dremel to carefully cut the inside of the plastic case panel. Using the clamp to hold the panel to the workbench, i carefully cut the circle on one side of the panel without going through the plastic completely. From there, i turned the panel over and redid the same cutting steps to cut out the hole completely. Then from there i used the sandpaper to even out the rough spots and used my fingernails to snap away any unwanted plastic.
I then took the metal power unit housing and hammered it flat against the concrete. Making it completely flat, i then cut it to 130mm square. After grinding away at the edges to they wouldn't cut anything, i placed and molded the metal to see how it would fit against the panel. After seeing how everything lined up, i then glued the 4 corners of the metal to the panel.

I then glued a small inner seam between the metal and the front of the panel.

After putting the screws back in, this is how it looks when the panel is on the computer.

Also prior to this mod, i made it so that all my internal fans except the top fans and the power supply fans are running at 7V. Even with the blow hole, i can barely hear my computer now. Nce and quiet, and nice and cool.