DMW = Daves Metal Works
I bought his 7 shot extension. Works nice. Fits 7 rounds in the tube, I used Fiocchi 2 3/4" buck and Remington 2 3/4" buck as well. some discussions mention that some brand of 2 3/4"shells will only fit 6 in the tube (Winchester and Federal come to mind if I recall the discussions correctly).
Based on this discussion link that someone posted earlier, I'm going to lose my clamp. I dont plan to be running around with my gun bumping into walls, rope, tree limbs, etc so I wont have to worry about the bumping my mag tube hard into anything. I mean the FN SLP is sold with an extension and it has no clamp on the barrel/mag tube so I would assume you can run mag extension tubes without any clamps and not encounter any problems (if you don't bump into anything).