I welded a 7-1/8" square plate to the chainring - sprocket
to give me more surface and a thicker base for attaching the box frame.
This will be inverted and the the box frame will attach to the other side of the plate you are looking at.
The sprocket is held in place by this base of the pedal shaft.
So I kept it and cut a hole in the plate for it to poke through.
Here the bottom bracket is disassembled.
This allows for bearings and cups to be serviced or replaced.
I trimmed down the pedal shaft ...
So when I mount the box frame to the top of this plate,
it won't interfere with the box.