I already own a 5 gallon nano tank, it has a built in overflow filter but that’s all
okay, If you are handy, you can build a Protein skimmer out of PVC or if you are really handy , A 2 liter soda bottle.
In reality tho, routine water changes should more than handle what you keep in a tank that small.
I never ran a skimmer in any of my small tanks. They take up too much space and remove plankton. And if there's only 2 small fish they won't produce much waste so just do a big water change every couple of weeks.