Help me identify this part

A little confused. Your info says you have a Gen II (2005) while the attached graphic is from a Gen III. Specifically, it appears to be the auxiliary box located beneath the "flying bridge" that everyone hated so much. The round hole on the right is for the auxiliary 12V socket and the square on the left that you have indicated is for the driver/passenger heated seat switches (if equipped). I snipped the page from my 2011 owners manual:

You might be able to order the blank that is installed in the non-equipped models, but I would bet you will have to order part #5 that comes with the blank. Otherwise, maybe an auto recycler pick-and-pull would have what you are looking for.

 

2015 Toyota Prius Cover. Spare switch hole; spare switch hole, no. 2. Black - 5553928160C0 - Genuine Toyota Part

If you live anywhere close to a junkyard and they have fairly recent Toyota vehicles, I'd take a look. Don't remember which car I got mine from. I had a air horn for a while and installed the switch in one of those, when I removed it, I was bothered with the hole in it, so I just grabbed another one at the scrapyard.