The Kx and Kr have the same body dimensions and I believe share the same body. I have an Arcatech K20 plate that fits on my K100 body just fine, and the body sizes differ substantially. Also, with an Acra-Swss compatible plate, you can use plates/clamps and brackets from any A-S compatible vendor -RRS, Acratech, Kirk, Markins, etc.
You can always go with just a flat plate, however one with a fitted lip will prevent the camera body from twisting. Personally, I would email all of the vendors and see what they recommend and suggest. Also, if you purchase through B&H, Amazon or Adorama you can always return or exchange.