A Rigid Buoyancy Devise ?

[tridacna]

The U.S. Navy plans to spend almost 1 billion dollars on a six man seal delivery device , what is wrong selling one of my own backyard builds at a discount of 999 million and 99 cents

Because yours is still close to a million dollars for backyard junk

 

[lowwall]

A rigid device as opposed to a lift bag, so the buoyancy characteristics don't change with depth.
Look for something like the housing of a DPV. Add weight as needed at the surface, seal the device, and dive. A DPV housing is designed to handle the crush pressure at it's rated operating depth.
I'd worry about a propane cylinder, as it is designed to contain internal pressure, not withstand external.

I'd use the DPV housing for the buoyancy, but simplify the whole thing by using external ballast to get to neutral instead of partially filling the housing. You could just epoxy some guides to the housing and use weight belts at the proof of concept stage. If it works, then you could rig up a rack or basket for the housing where you could easily add or subtract weights to get your load to neutral.

 

[Eastwest]

The navy seal museum is is only ten miles from my house . All the good stuff is free to see on the outside with beaches and parking , free free free and that aint no scam just ask google that !!