[knc1]

Or so I hope.

Image 1:

The PW-3 underside, the loose adapter, and the tool which will do the damage.
It is a square-end, end-mill, for a Dremel tool.
Claimed to be for soft metals.

Image 2:

The horizontal rib by the battery cut-out is cut down to about half its height.
The outer horizontal rib is cut down to approximately flush.

The center rib - that is a bit tricky - -
That "raised area" is actually the back of the e-ink display.
Stay away from cutting into that entire surface (both of the sides of the 'step').
Just cut the rib in that area 'close' but not flush.
The rest of the rib is cut approximately flush.

Image 3:

Which gives a 'home' for the adapter board, which is going to be a very close fit.
Zoom in on that picture and you will see just how close things are going to be.

The "Plan" is to sort-of pot that board in the opening (and maybe regain some of the e-ink frame's stiffness in the process of filling that area with a potting compound).