MB: ASUS P4SC-EA
CPU: Celeron 2.4
RAM: 512 MB DDR
SOUND: onboard
CASE: cardboard
DISK: 2.5" 80 GB IDE
PSU: Seasonic G-360
OS: Windows 98 SE

Pros:
- cheap, mobile and small
- one slot for pci and one for universal agp, nothing beyond the necessary
Cons:
- exposed
- not very quiet
- box's innards are awkward to access

I got the motherboard, cpu and ram for a few bucks; the rest are leftover components. The goal was to have a system to test various older video cards, mainly agp.

An existing case didn't have matching screw holes so I went the cardboard route. Everything is on or in the box except for the system fan. The hard drive is mounted vertically inside the box against one of its sides. Only the necessary psu cables stick out. Inside the box is mostly empty space so that some cut-up toilet paper cores can be planted for rigidity, esp. under the expansion slots. The motherboard sits on some silicone feet cut from a potholder.

A mouse micro switch acts as the power button, with a flap of rigid plastic over it to expand the clickable area. A 120-mm fan is mounted on the thing of an adjustable desk lamp that can swivel in about all directions. A zalman voltage adjuster is used to control the fan speed manually. Airflow is designed so that the fan cools about everything and the air generally blows away from the user. The cpu fan isn't used.

Ideally an ssd would be used instead of a regular drive but I don't have an adapter at this moment. I have the option to use a pentium 4 1.8a instead of the celeron but since performance beyond the celeron may not matter for the range of video cards I have in mind I might not bother switching it out. With something like an sfx form factor psu the system could be built into a smaller box but it doesn't matter much.