I was looking for a custom camera plate for the KP, and had no luck until I talked to Chris Hejnar at Hejnar Photo. I ended up sending him the battery grip for my KP as a template, and he sent me back a prototype (not anodized) to check out, and it looks good to me! Because the KP doesn’t have any good edges to hook on to avoid twisting, the plate has a pin that fits into the registration hole for the battery grip on the left side of the camera base plate. The depressions in the surface are to accommodate the little nibs on the KP baseplate, which otherwise keep the plate from making full contact with the baseplate of the camera. It looks and works good for me, so he is sending a batch of plates off for anodization. I recommend this product.
I receive no benefit from mentioning his work here, but as far as I know, this is the only custom arca-Swiss compatible camera plate being made for the KP, and it provides some real benefits over the generic ones I’ve tried. It’s $38, which is cheaper than some of the generic ones. Chris seems like a good, honest guy. Here is a link:
Pentax KP camera plate