Personally, to do something like that, I would visit a local resale, outlet, secondhand, or pawn shop and look for a commercially made surplus adapter of the correct dimensions that had visible/accessible screws to let you open it up. some need a special tool bit, which a little time with a High-speed rotary tool you might be able to make. Others that are sealed, with no screws visible, I've found they can come apart with little trouble using a rubber mallet on the seams. Or a regular hammer and a small piece of wood held on end against the seam. Sharp, quick blows on each of the four sides should break the seam free. Heavy blows will break or chip the plastic case, so should be avoided. Make sure to wear eye protection, and do it on a solid surface. I usually do mine on either the patio concrete or if the weather's bad, I keep a spare piece of marble tile around to do it on when inside.
Once it comes apart you can snip the wires going from the board or transformer to the AC plug blades. Dismount the rest of the old board from the case if needed, and voila! you have what you need. Use some superglue gel or two-part epoxy to seal it back up.
Hope this Helps,
aka David M.
"In Theory, the is no difference between practice and theory. In Practice, there is."