Checkout this tutorial from smittytone on how to make a Raspberry Pi retro restyle using a Sinclair ZX81.
After some searching on eBay, I found an old ZX81 going cheap because it lacked cables, though when it arrived, I found the computer itself to be in excellent condition. Possibly it has never been used, though how if that were the case the cables were lost and the box got so tatty is a mystery I will probably never solve.
Opening up the Sinclair machine was the work of a moment. Some screws are located under the computer’s rubber feet, so remove the latter carefully so you can put them back afterward – as was unlatching the keyboard connector ribbon and removing the motherboard.
One particular sign of age was the keyboard ribbon, which seemed in good condition at first. Unfortunately, 30-odd years is beyond the life expectancy of the plastic from which it was formed, and it soon began to perish and crack open. Eventually it provided so little protection for the delicate wiring, one line broke. Fortunately, I had a modern replacement. With a little gentle prying, I was able to lift the original membrane keyboard off the casing – it was originally attached with a self-adhesive pad – and replace it with the new one, ready to be stuck down in its place.
That was a relatively easy process; rather more tricky was getting the Pi and its cabling to fit inside the ZX81 casing. Fit it does, but only just, and I have had to make a couple of compromises.
My goal was to expose as many of the Pi’s ports as possible, at least among those I use. The analogue audio and composite-video jacks, for instance, have never been used, so I was happy for these to be hidden. But the HDMI, Ethernet, USB, GPIO and – obviously – power jack needed to be accessible.