Plates' dimensions matter more for lever QR clamps than for screw clamps.
I found that a bidirectional universal A/S-type camera plate does tend to shift some. One tends to want to overtighten the screw mounting the plate on the camera, and I would imagine that using a universal bidirectional plate on a 645z might require using Locktite to be at all feasible, given the size of the camera and lenses. I prefer custom-fit L brackets despite the increased cost. Of course, with the 645z having a side screw tap as well as the usual base screw tap, a custom L bracket ought to be an easy and simple design.