Camera rotating/slipping on L bracket in portrait

[GMac11]

Hi. Would anyone have any tips to prevent my camera from rotating/slipping down when in portrait orientation? I use an L bracket however I've noticed particularly when using the 70-200 it will very slowly begin to pivot downwards making shots of 10 seconds plus almost impossible to make.
I have tried different screws to hold the plate in place and have got them as hand tight as I can. I'm wary about over tightening in case I strip the threads or something.

 

[GMac11]

Camera body. Although I should say it's the f4 version of the lens so it's not all that heavy.

 

[GMac11]

It is pivoting around the screw for the l plate. It's definitely the plate as I first noticed it yesterday after I removed it from the tripod, the plate had definitely rotated a few degrees.
I have attached a couple of slim 3M double sided strips to sit between the plate and the camera body so I will see how that goes for a few days.