I am making a wetplate holder for a wholeplate tailboard camera I bought last year. As I'll be using only this camera for wetplates, I just measured the distance on one of the dryplate holders that came with the camera. If you don't have any for yours, then measure the distance from the back of the GG frame to the glass.
Unless you are manufacturing them for sale, you'll only need to make the holder to match your camera.
If you want it to be used on all your cameras, then make it to fit the largest dimention, and use thin packing to move the GG back on the cameras with a shorter dimention.