This is an old post, preserved for reference.
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I added a tactile switch keyboard option to the Minstrel last year, which looked very nice, but there was limited space around each key.
There was only space for one letter or number, no keywords or graphics characters. When I did a similar thing for the Commodore PET, the spacing was a little larger and there were no keywords to fit in, so I was able to fit the normal and shifted symbols in around each key.
I do have membrane overlays for the ZX80/81 which stick over a ZX81 membrane keyboard, in various colours and BASIC versions. As there is no visible switch element, there is space for all the artwork, keywords, symbols etc.
The new overlay PCBs combine the best of all those options.
I planned to use quite tall switches, but I found they were a little weird to type on, so I found some nice switches that were just tall enough to poke through with the PCB sitting directly on top of the switches.
The keyboards can also be ordered as an option to the main Minstrel 2 ZX80 clone page.
And Minstrel 3 ZX81 Compatible kit page.
2022 Update: At the time of writing, there are still a few Minstrel 2 (ZX80) and Minstrel 3 (ZX81) kits available from The Future Was 8 bit Those have this type of keyboard built in. But be quite as those are the Final Edition kits, there are unlikely to be any more.