Good in theory, not the most practical though - Magpul MIAD/MOE Core Storage

Using inside the Magpul K2+ grip. Getting these to open or close to start with is a pain. Not exactly something you could do quickly or consistently if you need access to whatever you're storing inside right away. Maybe it breaks in with time. I did have it pop out of my grip once after about 100 rounds, luckily I saw it and didn't leave it behind.

In theory it's a great idea to make use of empty space inside your grip. In practice it falls short.