You can use the design files from Outfab for a starter and modify them to meet your need.
What you need is 1/4in plywood and 1/4in acrylic, square nuts, 3/4in screws, mini ATX motherboard, hard drive ( I used a solid state drive), ATX power supply, 3 80mm fans and some polyester fiber for a filter.
Computer parts you can get from newegg and hardware from any ACE hardware store
Of course you need access to a laser cutter, or you can use an online laser cutting service.
Why laser cut a computer case, I hear you ask. Well, because I never could find what I was looking for.
I had several requirements:
1. Bottom intake, top exhaust (because heat rises)
2. Filter at the inlet, to keep everything clean
3. Higher pressure inside the case (fans at the inlet) that way you guarantee that no unfiltered air will enter through cracks
4. Slim, compact design that looks COOL
Step 1: Design
The original plywood thickness is 5.8mm, but after sanding it ends up being more like 5.6mm, that is why all finger joints assume 5.6mm thickness.
Step 2: Assembly
Everything fits like a puzzle, which then you fasten with the screws and the square nuts.
Now is the time to screw in all the standoffs for the motherboard. I did that by screwing a screw into the standoff, then using a drill to drive the standoff into the wood. Of course after screwing the standoff in, you remove the screw so you can put on the motherboard.
Step 3: Installation
All motherboards come with an IO back plate, which should fit in the huge side hole. You might want to measure that before you have it cut. Then install the motherboard, power supply and hard drive.
I tested the top PCIe slot and that worked fine, however the other two were off a bit, so If you intend on using them, please measure your motherboard or just Google for the ATX mini spec.
Step 4: Fans and Filter
After installing the fans, it is time for the filter, which in my case is just polyester fiber (teddy bear guts). You can get that from any sewing store. Stuff it in uniformly so there is no air gaps.
Step 5: Close the Lid
Step 6: DONE!
I put some LEDs that are not essential to the project, so I did not include them in the instructable.
You can get those from any computer store.
Also, don't forget the power button. I put mine at the top next to the drive.
You might wonder why there are two bumps at the top of the case, well those are to prevent people from stacking books and other things on top, blocking air flow.
I did not put a CD/DVD drive because I don't really use it more than once or twice a year.
I used an external drive for the installation.