I use a Greenlee jobsite box the same size as yours as my small arms locker (I am a low-budget operator and can't afford a safe ). I modified the inside to hold my rifle, shotgun, air pistol, ammo and personal papers. I have also found that large plastic food-storage containers from WalMart or Market Basket make excellent ammo storage containers. Their plastic construction makes them lightweight and inexpensive, as well as air-tight and waterproof.
Yeah, but they you're not meeting the CMR, the ammo has to be locked. It is a stupid rule and I could probably get away with not locking them. However, after all I went through to get my LTC, I'm not risking it.