Need help with Fan Speed Control

I'm new to the forum and new to electronics. I am using a 12V/80 Watt fan and want to use a switch to control high and low speeds. From what I've read it appears that using a diode (or multiple) would be the way to go but everything that I've read online has been in reference to CPU cooling fans, not larger fans drawing more amps. I need to mount the switch in a small plastic box so I think that heat might be an issue if I use resistors. Any ideas (written where a layman can understand) would be appreciated.

 

Thank you very much for the input. I've ordered but haven't yet received the fan so I'm not sure how much I can restrict the voltage and still start the fan. I understand that some fans need a minimum voltage to startup. Basically I'm just wanting to switch to a lower fan speed. The project is an ice-chest air conditioner to use in my airplane. Simple design - fan blows mass amounts of air into and over ice and cold air comes out of the tubes in the top of the ice chest. I also carry a 12V/7ah battery as a backup to run the unit before I start the engine, then switch to the aircraft power outlet. This is a new design and in the past I've used a much smaller motor that only draws 3-4 amps so switching wasn't necessary. This time I'd like to be able to run on low primarily when the small battery is powering the unit as well as for comfort control and to control the rate at which the ice melts (more air movement - faster ice melt). So, the desired result is less battery draw as well as slower fan speed. I'm open to any ideas.
Thanks,