I'll stick to my hole cube. :-)
Or trolley tokens (at least they're practical and get used)

Does all that for about a gram of plastic. And I really don't need more plastic models cluttering my workshop up.

I mean they could choose any thing, measure it and call it a calibration model.

Why not choose the secret heart box ?
If the hinges work - you're printer's dialled in. Job done. :-)