I have found that the Really Right Stuff and Kirk plates good in that they have an anti-rotation lip on them. Also I have found that plates with cork or rubber pads tend to allow movement. If protection for the camera base is wanted, maybe a very thin layer of rubber would reduce the tendency to move. At least one of the firms mentioned has an L-shaped plate for the Mamiya 7, or alternatively and maybe better long term, a general L-shaped plate so it can be used on future cameras. Downside might be lack of access to the base plate for film loading.