Hiding Spot Theory 101
First, you should always be looking for things that can be made into hiding spots. Do this at home, at school, at work, while you're on the john, etc. The reason for this is that it helps build your mind. You want to be sharp, observant, and quick thinking. You should also find that this will help you build and find better hiding spots.
Second, always be on the lookout for materials that you could use. If you see any take them. Some ideas to think about, anything that has a latch, clasp, or lid is worth saving. Think shampoo bottles, food containers, shoes, old backpacks, belts, toys, etc.
Lastly, once you come up with an idea and the materials, build it. It doesn't matter if you don't have something to hide yet, making hiding spots is great mental exercise, and it is ton of fun. And you might even be able to sell your home made hiding spots to people!
Always consider the size of the object you are hiding, the reason you are hiding it, and who you are hiding it from. Make sure what you are attempting to hide will fit into what you build. If you are simply hiding personal items that you'd rather not have people know about, such as letters and questionable material, the hiding spot doesn't need to be too elaborate. If your hiding illegal things you obviously want to get as elaborate as possible. If your hiding things from say, your parents, you might get away by hiding stuff in things such as cd cases or books they would never read. If your trying to hide something from everyone you will need to spend considerably more time in the concept and design stage. Finally, make sure you're not constructing a cliche. Examples are, under a drawer, behind a clock, under bed, and the worst spot you could think of, a hollowed out book.
There are three more design concepts to think about. Make sure it is inconspicuous, still functional, and make sure that it can't accidentally be opened. Many things are inconspicuous, books, cds, pencil cases, etc. Idealy whatever you turn into a hiding spot should still work, like a shampoo bottle with some shampoo left in it, or a box with a false bottom. And lastly make sure and can't just be accidently opened, this is where you need those locks and clasps. You will need to build them into your hiding spot.
The most important thing is to be careful and precise. Any mistake can give away your hiding spot, be extra careful while cutting and gluing. The second is to make sure no one sees you building it, as this will compromise the whole project. And lastly, make sure you clean up all the materials you used, and leftover bits. These could tip someone off to your activities and make them more alert than normal.
Be sure to have fun making your hiding spots, if you think of anything /msg me!