BSA Martini side mount for scope.

I'm going to make one of these.
But if you look at the photo mine will be longer at the front and keyed down to that rear barrel mount. Might be as simple as remove that barrel wedge and use a longer screw to attach to that rear barrel screw position.
But...would 6mm angle aluminium be thick enough, or go to 8mm?
Obviously there is a length of Picatinny rail screwed to the top.
I'm thinking 8mm but am I going too big?
Thanks for any ideas, etc.

Attachment 271346