$2 | Old briefcase (Goodwill)
$15 | AC to 12 & 5v DC 4pin Molex 2A Power Adapter (Amazon)
$4 | 2 x 80mm Case Fans ( Pick any fan) example fan (Newegg)
$ ? | 4 x 80mm Case Fan Grills (pick any matching grill) - (Google Image Search)
Note: If you are going to be installing a computer (motherboard, power supply (PSU), etc.) you will not have a need for the $15 AC to DC Molex Power Adapter as the fans will connect directly to the PSU. Additionally, for any project you need a minimum of 1 Grill for each fan, but I recommend buying 1 more in-expensive grill to attach to the fan on the inside of the case. This will protect your cables and fingers from any accidental hiccups.
Step 1: Gut and Cut
When you have the holes close to the size of the fan's inner diameter, get ready to attach them. You may have to pinch starter holes for the fan screws to penetrate. Additionally, this is a good time to choose your air flow directions. I recommend one pushing air in and one pulling air out (just to keep things flowing).
After the Fans are installed the way your like, use a little bit a tape and control the wires. Feel free to stick in the AC to DC adapter.
If you plan on installing a mother board, line up and use a dremel to cut a side in the case for external connectivity. I just cut holes to run cables in and out.
Step 2: So Simple
And that's it. It's much simpler than it looks so don't be afraid to get a couple cases and get creative with it.