You could start mating them one by one and can orient in best possible way while you mate.
I would use Bounding Box to determine the optimal orientation for your parts. You can even use the elements of the Bounding Box to mate to the bottom plane of your box.
To create the Bounding Box:
"Insert" - "Weldments" - "Weldment"
Right+click "Cut list" (almost at top of the feature tree) and select "Update"
Right+click "Cut list" again and select "Create Bounding Box"
Unfold the "Cut list" (the small "plus" button) and you will se the Bounding Box as a hidden 3DSketch. Unhide this and it will show your optimal orientation.
Btw. Don't send money. Just mark answer as correct (if you think it is). I want this forum to be non-profit and just helpful.
Wow, that is great! Thank you so much for the help!