Installing autometer tach or speedo into stock cluster?

[Ieatcamaros]

why can't I just make a plastic trim piece with a round hole?

There is no reason, you can't. You will just have to take it apart and make something work. Sounds mean, but there is not much else to say here. All you will need is some good sturdy plastic to cut and fit into place. Of course, you could just mount your 5" tach on the right side of the gauge cluster on the dash like everyone else. I got the Autometer gauge pod to hold a 5" tach and (2) 1 5/8" gauges. The pod cost like $50 from summit. To each his own.