My first question would be, what are you planning on pulling with it? a lightly loaded 1man shack vs a loaded 3 man or a chain of shacks would change how I would mount the hitch. If you are going to pull only light stuff and the auger mount is secured well to the bumper, I would consider mounting the hitch to the digger mount. If it you are going to pull heavy loads I think you should consider drilling and mounting to the bumper.
Another option is to design a bottom plate for securing the digger mount to the bumper and incorporate the hitch into the bottom plate, similar to the plate already used.
After thinking more while typing this, I would mount it directly to the digger mount, and add a spacer block that spans the width of the inside of the bumper so that the force is transferred to the bumper during pulling compared to the 4 contact points in the current digger mount clamp plate.