Adding Batteries

[Viper]

As long as there is the ability for air to circulate, you don't really need an additional fan. Add openings so that air can circulate by convection in and out of the space and you'll be fine. You're on the right track with openings at different heights. The caution here even with the addition of a fan is that you ensure the compartment not be used as a catch-all storage space where things can fall/shift and block off venting.