Most 2" adapters attach to the same reducing ring as the stock 1.25" visual back. This does not provide a true 2" aperture. For a full 2" aperture you must remove the reducing ring and attach the 2" adapter directly to the 3.25" opening on the rear cell.
Celestron offers a '2 inch Visual Back'; their part number 93792 that attaches to the 3.25" opening. This part can be found at most Celestron dealers. This is a basic unit with set screws that thread directly through to the inside of the barrel rather than utilizing a compression ring that protects the barrel of the diagonal.
Astro-Physics offers an adapter that threads to the larger 3.25" mounting ring, eliminating the reducing ring and providing a full 2" aperture. Additionally, the thumbscrews are behind a brass compression ring that provides better security with no marring of your 2" diagonal. Check out 'Focusers, Adapters and Extensions' in the products section of their web site and be sure to get part number ADASCTLC for Celestron scopes:
Baader Planetarium produces 2" adapters for the 3.25" opening. My first choice uses the 2" Clicklock eyepiece holder - you can get an idea of how that works in my review of the Clicklock 2" Dielectric Mirror Diagonal here. The 2" Clicklock eyepiece holder for the 3.25" opening is Baader part number CLSCL-2.
Another option is Baader's traditional eyepiece holder with two thumbscrews. The thumbscrews are behind a brass compression ring that provides better security with no marring of your 2" diagonal. You would need the 2" eyepiece holder and an adapter for the 3.25" opening - part numbers T2-17 and T2-18.
Details and purchasing info for these Baader items is found here:
Note that Peterson Engineering no longer produces the EyeOpener for Celestron scopes as mentioned in my book.