I dont own one but you may contact MTM Ammunition box charts for rifles and handguns by Case-Gard . They do make a 10 round slip top for 50 BMG, part # BMG10 and a shotshell case they claim will hold 50 BMG50 rounds, part # SF1000. It cant hurt to send them an email.
Sorry to resurrect an old thread, but I spent hours trying to find a good solution for carrying Cheytac ammo. Only to find out that the Case Gard H-50-XL works just fine. I did like the idea posted above about using the 20 gauge shotshell tray for reloading purposes. All of the .50 cal and shotshell ammo boxes are so much more bulky than you need, the H-50-XL is perfect.