Paul,
A good source of short lengths of thick wall aluminum tubing is old aluminum SCUBA tanks. I made a housing for a camera that has been to 1000 feet in the ocean and 2000 FSW in a test chamber from a 6351 alloy tank that was free.
Yes, good idea. 2000 FSW is pretty impressive. Unfortunately the majority of tanks here in the UK a steel so it rare to see aluminium tanks that have failed a test (or at all) here. Techies use them for deco cylinders as they have better buoyancy characteristics. I might ask around the local test facilities and see if they ever scrap them.
Paul.