I like Doug Ritter’s Pocket Survival Pak. I’ve carried one in my back pocket for years. The plastic pouch is flexible enough to ride comfortably—though eventually I’ve had them give out and need to be replaced. You can open them up to add stuff of your own. Water bag, water purification pills, and I replaced the golf pencil with a Fisher Bullet Space Pen.

With those sardine can kits, it’s either open or closed. You have no flexibility.