The Perixx PS/2 keyboard suggested in the Parts List (PERIBOARD-409) is built to be used on desk and as such has a nice casing already.

However, we want to put it in our custom case, where only the top will be visible.

So, we need to :

1. Place the keyboard face-down on a soft flat worksurface.
2. Unscrew 11 crosshead screws from the back of the keyboard. Note that there is a screw under the "Q.C. PASSED" sticker and one more under the big "perixx" sticker, under the square-logo, to the left of the FCC symbol, near the bottom.
3. Remove the back cover by prying open the clips located at either side, roughly in line with the TAB and ENTER keys, using a small, flat screwdriver.
4. Get a rectangular piece of 2-3mm thick perspex / acrylic / wood, cut to approximately 30cm (1ft) long by 10cm (4in) wide. IF YOU DON'T HAVE ANY MATERIAL TO USE FOR THIS STEP then just cut out the large flat rectangular section of the keyboard back cover and skip step 5.
5. Using the base cover of the keyboard as a guide, mark out and drill the three holes that run across the middle of the large flat area in the case, onto the perspex / acrylic / wood.
6. Screw this prepared piece of perspex / acrylic / wood onto the back of the keyboard, such that it holds the membrane switch circuits in place, like the back used to.

Optionally, if you have a soldering iron, etc. or some small screw terminal blocks, you can also shorten the keyboard's PS/2 cable.

Simply cut the unwanted length of cable from between the PS/2 plug end and the keyboard controller end. Make sure to leave enough cable either side for you make the wiring connect. Maybe 15cm (6in) or so.

Strip back the sheath and insulation from the two ends of cable and join the same coloured wires to each other, using your preferred tools / method. Be certain to use heatshrink or insulating tape to prevent short circuits between joints.