I'd go the cheaper route and use anti static foam like this https://www.elexp.com/0302asfanti-static-foam-4-x-5.html and cut rectangles which fit the cardboard box. This stuff is available in huhe sheets and is quite cheap, too.

3D-printing is a waste of time and money for stuff like this in my opinion :)

Thanks for suggestion. I already thought about using antistatic foam. one additional benefit of this would be that I could place 2/3/4 devices in the same box without needing a new tray.
Reasons for the pros for the print so far are:
- Not expensive in this case. Third tray only needs 0.06g/ (sub cent cost if you use cheap filament) And Left over short filament pieces can get used.

- More unique

Time wise I am not sure if getting the prints off the printer takes more time than cutting the foam to size. But maybe the foam can be laser-cut to get a lot of little rectangles at a time.

What I like the most whether using foam or trays. I can switch between both at any time so far without the need to change the other parts of the packaging.

Yeah, you're right! Letting the printer do its work sometimes really is cheaper than ordering stuff + postage and packaging.

And for a few dozen prototypes it really doesn't matter all that much. Best of luck selling those beauties!