Hidden doors -- This company makes cabinets, bookshelves and anything you can think of that fits your existing doorjambs thus hiding the door (and also making use of the space) Not sure I like the idea of a 'gun cabinet' door.
You can make this awesome and functional pallet wood project all by yourself. We have made a dustbin in this project. You have to make various sections in this pallet wood project. We have painted this project in brown shade.