Is that 42" length with a fixed or collapsible stock? If collapsible, can it go shorter? Blackhawk makes a 'discreet' case, which will be plenty tall for an AR platform. But it appears the longest is a 40".

https://www.amazon.com/BLACKHAWK-Homeland-Security-Discreet-Weapons/dp/B001EC1WIW/

I run the 32" version for my 16" AR (collapsible stock).