based on what I have read here, I ordered, and received today, a Joby Ultrafit hand strap with Ultra Plate from Amazon.
https://www.amazon.com/JOBY-UltraFit-Hand-Strap-UltraPlate/dp/B00F5JHDZG/ref...oby+hand+strap
I am sure that other retailers may carry it as well.
I attached the ultra plate to the baseplate of the Pentax K 3"s battery grip and the hand strap to the same attachment point on the top right side of the Pentax K 3 where I also have attached the Optech Utility sling. I am right handed and this allows me to wear the sling on the left side of my neck and lets the camera slide up in front of me as I pull it into shooting position with my right hand. It hangs off my right hip when I am not using it.
The systems seems to fit well together. The biggest problem I had was getting the straps through the necessary slots (I have thick fingers and they are not very nimble.)
The only downside I foresee now is trying to keep track of the allen wrench necessary to attach the ultra plate to the base plate of the camera's battery grip.
the extra threaded holes in the ultra plate allows me to mount the system to a tripod or monopod without having to take the hand strap off and I can quickly detach the Utility sling if necessary when needed.
I chose the Utra hand strap with the ultra plate because I figured that if it didn't work as I hoped, I would at least have ultra plate to use with the tripod or monopod.