I was dropping off a H.V. probe to an amateur radio friend of mine. He was rebuilding a Heathkit SB200 amplifier. I had recently done the same.  In our conversation we started discussing Raspberry Pi. He had killed one a Pi 4B (4GB) by putting 9V on the GPIO power pins. It was toast. He had a 2nd working Raspberry Pi 4B (8GB) in an Argon case. After a couple days I decided that it might make a good fit in my HX-20. A deal was made. 

I had no idea if it would fit. I don't like to be bothered with measuring things.

I would say that's a damn near perfect fit.