This is another super specific design that very few (if any) other people will need. TL;DR: I have a gameroomsolutions pinball cabinet with the triangle plunger cut, and bought a zebsboards v5 plunger. This should fill in the triangle cut with the required holes.

The zebsboards plunger apparently doesn't use the triangle cut anymore (even though the manual says to cut it, which is why I ordered the cab from GRS with that cut). The cabinet just needs a hole for the plunger shaft, and a hole to screw in the faceplate.

I modeled this in fusion 360 but the triangle cut is somewhat irregular. Instead of using measurements I took a picture of the hole with my phone and traced the spline tool over the top of the cut. This mostly worked, but the plug ended up just slightly oversized. Instead of wasting even more time trying to re-model it, I decided to just grab a rubber mallet and hammer it in, which worked fine. If someone else were to print this they may want to consider scaling it down to 98% or something. Use at your own risk of course. Don't ruin your cabinet.

I just made this for a quick and dirty fix and then threw out the design. Free copies of Fusion 360 only allow 10 editable projects, and I didn't want to waste a slot on this thing. I've included both an stl and a 3mf. Feel free to remix it.