There are different material data needs for static analysis compared with dynamic analysis for an impact study. In statics, rate-dependent properties are unimportant, but in an impact problem, rate-dependent properties are very important. While there may be a negative slope (softening) in a quasi-static material test, there may be a much higher stiffness during impact.
ANSYS Help in the Mechanical APDL section has a Material Reference section and Chapter 4.5 is on Rate-Dependent Plasticity material models. Take a look at some of those. Perhaps others will reply with advice on which one to choose.
Take a look at the Explicit Dynamics material library. There are material models called Equation of State. I don't know how to create that kind of material model, but I see it is used frequently in the Explicit Dynamics material library.
What are the design requirements for the drop box? What is the peak acceleration that you are trying to stay below during impact? What is the mass inside the box?