I have always used the ammo boxes that the ammo comes in, but no more. My wife notices some great pouches that people were using to hold ammo and was told they only come with a particular range bag. And the manufacturer does not sell them separate.
After much search, I found these. These are basically the same, but I don't use them as a brass pouch, I use them to hold rounds to reload my magazines.
They are just the right size to fit most range bags, can be slipped out of the bag, very stable (compared to balancing a box on your leg) and have a generous diameter so your hand can easily reach in to grab your rounds. Even has a clip so you can take extra rounds to the line if you need to.
These are now permanent equipment whenever I go to the range.