This 12" Double Ball Buoyancy Arm by Ultra Light Control Systems were designed for the heavy Digital SLR housings and video housing.
They are 2" in diameter and constructed the same as the regular buoyancy arms. These arms have 2.5 times the amount of buoyancy over regular buoyancy arms. You will need to use the AC-CSB in order to attach more arms for more buoyancy force. The 8" arm will provide you with 11oz of buoyancy force.