Thanks.  I like the trig one better, because I understand it :-)

I don't understand cross products, and I only know how to construct
transformation matrixes from recipes.

There's another variation of the trig answer in the OpenSCAD docuemtnation:
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Transformations#Rotation_rule_help

End of the line now. Multmatrix doesn’t work with difference()…