This is an old post, preserved for reference.
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Ben Heck has recently been building a Spectrum portable computer (that's, a ZED X Spectrum by the way). In that, he used a matrix of tact switches for the keyboard. I remember trying something similar a while ago, for the Spectrum+. I took some photos, but didn't bother writing it up. I thought I may as well do that now.
designed a replacement membrane.