Thanks Adhoc, I'm adding a bezel with sorbothane to seal the gap round the speaker once in position so the opening I have should alright. In sketchup it looks perfect, in real life I'm sure its probably going to be a real pain, dreading it and excited at the same time! Most of my engineer friends think I'm mad using springs but I think I'm on the path to sonic nirvana and if it doesn't work I can always remove them.
Do you think the speakers would benefit from being clamped using the same system from the sides as well or does the top system eliminate all noticeable movement?