That's right, the plate has to be free to slide & mine is. It'll find its own optimum position & no doubt sit there. I can't see any point in securing it, plus it would be a complete bastard of a job to do.
There is absolutely no doubt that the plate should move...I set mine up in much the same way as you have, loose enough to move freely but not rattling about...
The plate has to move when the primary chain is adjusted and it would take a contortionist with magic spanners to loosen it and then to tighten it again afterwards...:laughing: ....Ian