I think the easiest way would be to bolt a small dovetail to the bottom of a piece of wood. put a 1/4" 20 bolt through to attach the camera and a locking bolt so you can screw the bolt up into the camera...
With you're 70-200 lens you should be able to get 10 minute unguided shots with a very precise polar alignment. 5 minute subs should be easily attainable...
Good luck mate, and welcome to deep sky imaging!
I hope your wallet is prepared for what you're about to do!!