When I planned my project I've looked for info about dimensions in the internet, but not found exact sizes.
I've loaded lot of photos of the Baron cockpit, then calculated all sizes in the graphic editor using all well known dimensions, such as cutouts for 3 1/8" instruments, sizes of the GPS, ADF etc.
So I got all sizes that, I believe very close to original (+/- 1cm at whole panel width, or 1%).
Of course, if I had to make these pens now, I could use a 3D printer, but when I started this panel, I didn't have as many options. So, I used everything I could find and that can work as knobs:
For the smallest knobs (used as GPS, ADF, COM/NAV volume control) might be used any suitable cylindrical parts (caps from pens or casing from some electrical connectors as in my case)
Another method was used for making knobs for the transponder, altimeter and ADF Indicator. The knobs of suitable diameter from some old cassette player. They have smooth surface, so I've graved notches with simple "router", made (that took about an hour) from the monitor stand and drill.
And, for some knobs would perfectly fit the caps for tubes of glue, creams etc. If diameter is not fit, some of them could be cut and glued for needed size.