I have a straight edge tarp that functions well. It does require an extra stake in the center. However it has its advantages.
Its better suited as a ground shelter than many other cat tarps. In zero degree weather, i dont use my hammock for weight concerns and the cost of adequate insulation.
I have beaks or endcaps on it that help protect from blowing rain, but a cat tarp can have the same.
The extra stake out in the center helps distribute the tension of the other corners, and helps pull the tarp away from the hammock.
So while the cat tarps have their advantages, they are not the only good choice.