spartacus said..
Any suggestions, given the soft foam of SUP's to work with?
[...]the amount of force the metre long mast & wings create.
You want to anchor firmly the box into both the deck and hull skins. Forget the foam, it will not be strong enough.
And reinforce heavily the skins, especially if they are not already real sandwich.
I would add strap inserts.
And I definitely would find a shorter mast. Hitting the bottom with the 1m mast is a high risk in waves, plus the insane torque any water movement created by the wave and various rips will apply to it