The revised plan was to use separate ZX Spectrum USB keyboards and mount the Raspberry Pi's behind the scenes. I had a look around for suitable wall mounting Raspberry Pi cases, but was surprised to see there weren't many around.
A fried of mine found some 3D models on Thingiverse, Credit to the original designer these were very nice cases. The lid was a bit busy with cutouts for vents, camera, monitor, GPIO etc. so he respun the design to have a plain lid, then added the 'ts' tynemouth software logo to give a bit of ventilation (see his write up of using Fusion 360 for this). He then 3D printed the cases for me.
The mounting holes are on 100mm and 75mm squares, so are VESA compatible if you wanted to mount one on the 4 screw holes on the back of a monitor. These looked great with the activity lights shining through the clear cases.
The Spectrum and Commodore 64 emulators run fine on the Pi Zero, so I used the single core Raspberry Pi B+ for the other units to share the same microSD card images.
Later a 1990's PC game was added to the exhibit which needed a bit more power to emulate, so I used a Pi 2. I did try the Pi 3, but in this situation the onboard wifi was a disadvantage, and the extra speed didn't make a noticeable difference.
The Pi 2 cases were printed in blue to distinguish them from the Pi B+ ones. It is the same 3D model, there is enough space and vents on the base of the case for cooling of the extra chips on the 2. This was purely to make sure the units didn't get mixed up.