If you look at the original Fish traps made by Dave Genz, you can see that these things are pretty basic. The hoops are conduit and the hinges are made with bolts and a piece of sheet metal with holes. You can easily build one, but you will not save any money, unless you can build it out of parts you have.
If you just enjoy tinkering, go look at the models in the store, or find the Dave Genz documentary on YouTube and study the old version fish traps.