I do not think that the existing K3 keyboard plug (I think Seaniko7 was making some cables), does not take enabling power into account. The bottom slot has four pins. The bottom two pins need to be shorted together to enable power to the top slot.
To supply power to external devices, the K3 needs something plugged into both slots on the left side.
Ordinary PCB material is too thick and needs to be made thinner to fit the kindle slots.
Another way may be to get some copper foil tape from a craft supply store and stock small pieces of copper tape on thin plastic such as from an old credit card. You know, the "hacker" way.
You could find out where you need to cut by sticking fresh shiny copper on a card into the slot, and see where the pins leave scracthes or dents on the copper, the separate those traces. You can solder to copper foil.
A agree about the eInk display being kind on the eyes (especially outdoors). I also agree that typing on a "full-size" QWERTY keyboard is much faster than on tiny chicklet-style or onscreen keyboards.
I believe that you can get "real work" done on the K3 plus external keyboard combination. I look forward to working outside in the sunlight with no eye strain next summer.