For a truly unique cajon building one from scratch is the way to go. Although there are many variations and options you could include when building a cajon, in it’s basic form a cajon is simply a wooden box.
There are many examples and guides around to help you build a cajon, some in lots of detail and others with less detail that can serve as inspiration and ideas.
Here are some instructions, example plans and builds that might help in building your own. If you want to build a cajon, but doing it from scratch sounds like too much to start with, why not try a cajon construction kit.