[High School Calculus AB] Assistance in finding an idea for creating a related rate project.

My teacher assigned the class a related rate shoe box problem. Basically I'm to create a solvable related rate problem that can be modeled in an average sized shoe box (I can create a a box out of any material, as long as the size is similar). I was thinking about creating a zombie apocalypse scenario where the rate a zombie infection is increasing. Now the problem needs to have some levels of difficulty so I don't think just solving for a dx/dt will get me an "A" (Project counts for 2 tests grades)

I found this page http://dsmcdaniel.tripod.com/Assignment_6.htm as a simple guide, but I'm looking for more input and difficulty. Maybe I could use a sphere (as in earth and use real measurements?) instead of a circle. Any suggestions?