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.