You don't even need a styrofoam cooler, just a plate of glass and some cardboard. Put some insulation around the outside, I've even used more cardboard.
Beeswax melts at 146°F (63°C), so you don't need a lot of sun. On a good sunny, dry day, it will easily get around 225°F (107°C). You don't need to get it hotter than that, the wax will degrade.