I think you'd best make one. Retailers hear the word 'Barrett' and think your name is Trump. All of 'em seem to require disassembly too.
However, Pelican makes a 'Storm' that's 54.5" inside. Their number IM3410. Comes with or without foam.
Cyber, Texas doesn't appear on their dealer locator. snicker. Lots of Pelican retailers in Texas though. No idea what they want for it. http://www.pelican.com/us/en/pro/product/watertight-storm-hard-cases/large-case/long-case/iM3410/