timer for light switch in basement

I have a "light switch" in the basement that turns on and off the power to an electrical outlet I have outside my house. I use this outlet to power my Christmas lights.

I want to replace the switch with a timer that also has a switch. That would be easy for a regular house light switch, but I am unsure if I need a special timer/switch.

First, the switch box is big. It looks similar to this Switch Box Factory. To me, it looks like you can put two switches in this box. However, there is only one switch and its centered. Secondly the plate is metal. The switch is outside of the concrete wall, so its not a common interior switch.

So if I buy a timer like : Honeywell 7-Day Programmable Timer - Wall Timer Switches - Amazon.com , I don't see how I can install it in the center of that box. Or is it that easy? Is it just a matter of using a particular center plate?

 

You can use a 4" square single opening like in the picture below.

 

You only need to change the cover on the existing box to one that has a cutout for a GFI or Decora switch.

4” Square, 1/2" Raised Decorative or GFCI Receptacle Industrial Surface Cover