My first thought was "crazy idea" but the rigid drum actually opens the door to a nice feature.
What is the big nuisance with a soft lift bag? Expanding gas volume and the resulting increase in buoyancy. Not a big problem if we want to send something to the surface. But if we want to relocate a heavy object underwater, that expansion is going to make our life really miserable. It creates an unstable equilibrium that we have to fight with venting and addition of gas.
Now, picture a water-filled drum, underwater, with the two holes facing down. One hole is closed or has a one way valve for injecting gas. The other hole is sealed with a pipe that goes up into the drum to, let's say, half of the height. The end of this 'standpipe' outside of the drum is open.
If we would fill this drum with gas until the gas comes out of the standpipe the buoyancy (approx. 220 lbs in this example) would remain constant on the way up. Any increase in gas volume due to expansion would just vent out of the standpipe. Bingo - constant buoyancy lift 'bag'. (At least on the way up. When going back down, we would have to add gas to compensate for the compression.)
If we make the standpipe sliding in a bung fitting we can adjust the buoyancy to any amount we wish.