I'm a great one for making what I need ... but somethings are better left to the 'factories' and this is one.
Skyartec produce a battery box that is sold as a spare part if needed ... plastic vented frame with hinged door and also reasonable push in undercarraige slot if wanted ...
Cost ? Under $3 + shipping. Available direct from Skyartec outlets or good ol' Hobby King.
The box will take any size up to 2200 3S standard packs and is secure.