Mine works on breadboard, you need to find a sturdy box. GEOFEX has some ideas for using steel building materials. Small Bear has switches which look good. Relays are the non-latching type 5V NEC MINIATURE LOW SIGNAL RELAYS, Mouser Part#:

551-EA2-5NJ, Mfgr. Part# EA2-5NJ .  Transistor are generic things like 2N5088. Diodes 1N4148. You need a 4MHz crystal oscillator and a PIC18F877. I am using a PIC18F877-20P which is 20MHz, the 4MHz version would be fine. It is static sensitive, and has 40 breakable pins.

The microcontroller chip is a 16F84, which is static sensitive. I have the program for it written, you need fairly simple hardware to transfer the information from your computer's parallel port after you download it. Not really simple for most people, so I might sell some preprogrammed chips.

The relays can be in a remote box. I will get some form of parts list going.

Cheating: This cost me $40:

It has 12 footswitches and I think I can run my relay switcher at the same time as the existing MIDI electronics. So I can use the LCD display. The relays would be in a separate box, but you need 13 wires for 12 relays. Later on I might do a version with relays and MIDI in, so I can use any unmodified MIDI controller with a MIDI cable running to the box.

You can get a big box and put in your own switches.

Each relay uses 140mW of power when activated, 1.68W for 12. 0.336A at 5V. So a 5W AC adapter would proably be minimum standard. A 7805 voltage regulator has a maximum of 1A output, so that should provide the necessary 5V for the 5V relays, with a nice big heatsink. Microcontrollers need a regulated 5V power supply, so one 9V wall wart and one 7805 with 2 capacitors gets us there.

This might do the job:  "Radio Shack Digital Camera Power Supply"  3-6.5VDC/2500mA AC-to-DC Adapter $19.99   Reg. Price Brand: RadioShack Cat.#: 273-1695