Here are two DIY options that act like a button press (digital) and a 3rd option that is analog.
DIY option 1:Many builds have used a leaf or microswitch with a plunger, but the switch can take a pretty bad beating, depending on how and where it is mounted.
IIRC one build used a fender washer mounted on the tip of the plunger using a cotter pin or two through the plunger shaft.
When the plunger was pulled back, the washer would press the lever arm of the microswitch until the plunger was released.
There were also blocks that kept the washer from pulling back so far that the switch would be damaged.
Couldn't fnd any photos of that build, but it was somewhat like this diagram.

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DIY option 2:An optical circuit like
this.
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Option 3:Last time I looked on the
VP Forums, the
VirtuaPin™ Digital Plunger Kit v3 was a pretty good option.

Scott