I've been working on restoring some of my 80's computers, one of which is a TI-99/4A. My particular one has a Hi-Tek High Profile keyboard in it, though there were several other types of keyboards, including a popular Mitsumi that used a membrane.
In taking mine apart I found several broken plungers. After looking all over, I couldn't find any source for replacing these plungers / switches other than cannibalizing old hardware, so I decided to see if I could 3D print one.
I've had some amount of success, though my 3D printer's tolerances aren't quite good enough to print this without a fair bit of sanding and scraping of the print and PLA / ABS are not the ideal materials for this (the original plunger looks to me like it might be nylon). However, it does seem to work.
In my current test case I didn't do a good job of cleaning the lower inner spring housing and I think the inner cross bar is slightly thicker than the original. Both of these make the travel scratchier than the original, but that should be fixable with a bit more cleanup.
If there's any interest, and after a little more refinement I'd be happy to put the FreeCAD file up for others to use. The part will print without any support structures, so if you have a 3D printer it's mostly a matter of sanding and scraping and how much will depend on your 3D printer's capabilities.