Id say Winky Wright and Joshua Clotty are the masters of it. Dosen't make them exciting fighters and has caused Clotty to come up short in big fights but has saved them from a lot of punishment.
Clottey has come up short due to his workrate. Wright could fight in the turtle shell but still be agressive sometimes like he was with Mosley & Vargas at times. Kotelnik also uses the turtle shell defense but like Clottey, he too doesn't have the highest workrate. Quartey & Marlon Starling used it too.
It's a tough style to master. Winky Wright and Bronco McKart in their respective primes did it about as well as anyone I've ever seen. Both are southpaws with good jabs who liked to exert pressure whenever possible. I don't think that is a mere coincidence.
Most fighters just don't have strong enough forearms to make the turtle shell work for them. Doesn't help you much if by blocking the punches you're just punching yourself in the face. It's worked very well for some very good fighters, just it's tough to do, and you still need very good quickness to be aggressive while in the shell.
The bold part is a perfect description of Arthur Abraham. The day after a fight he generally looks like some one beat him in the face with a brick!
Manny Pacquiao couldn't break through Clottys turtle shell any more than Zab Judah could but Pac solved the problem somewhat by body punching Clotty effectivly. One thing about Clotty he is hard to look good against. Its just hard to look good against somebody who fights out of a turtle shell.
