Wine crates can be used in many DIY projects. They are usually made of solid wood but are really inexpensive. Probably you even have several of them lying around but if not you can always buy them in local wine shops.
Here is a bathroom storage cabinet made of two wine crates. The bigger one is attached to the wall with screws while the smaller one is screwed into the first crate. As you can see, two crates are more than enough to create a functional cabinet with shelves.