Fin box installation channeled Bottom?

Planning to build a Tomo like Board, I´m struggeling installing Future Fin boxes because of a double concave, channeled Bottom.

Should I install the boxes before shaping the flat surface, or would it be better to create the Bottom Contour first, install and sand the Boxes down to fit the Foam after?

 

What kind of box? 

I would be inclined to install post glass. Much easier to sane the boxes smooth.

Some people don’t like this and may have a better way of doing it.