we were told by our health and safety expert that signs were enough as long as there is a picture on there so the illiterate yobs can get the picture if they can’t read the no unauthorised use etc ,
he explained that putting a shield ie plank made it more of a hazard as people would still attempt to climb but more likely to fall off which actually makes sense,
I would suggest this to your land owner before investing in a lot of locks and safety boards ,
I have over 50 high seats and none have these planks etc just clear signage , up to date high seat register with photos of all seats with appropriate signs on them .