Mushroom insert securing bolt

[voucht]

Hi,
I know that in the UK, you have to put a bolt on the side the front upright in order to stop the mushroom insert tu turn and secure it. I don't have to do that here, but I really like the idea and I'm going to do it anyway. But I'm not really sure where to drill.
Where is best place for this bolt? what size shpud it be? (I was thinking about M6, as I have a M6 tap)?
Any advice (and picture to illustrate ) are welcome.
Thanks.