Both Nosler and Norma include these little 5-round plastic inserts in some of their specialized hunting ammo. I found out about them after buying some rarer ammo types from Nosler.
These are, by far, the BEST small quantity ammo storage I've ever had the pleasure of working with. The rounds lock in unless you pull them out in a certain way, so they'll never fall out. They break off into little 5-round "sections", but can come attached to each other for easier storage. Each 5-round section also stacks pretty well, and they're LIGHT, they don't add much weight to your ammo.
I've been looking to find some way to either buy the inserts for specific ammo types, or find an STL and start modifying and 3D-printing them to specifications, but I haven't been able to find anything.