There are three different sized fasteners used for those under body panels. The ones that are most likely missing are easy to find at a Toyota dealer.
If you're near me in San Diego, I have them in stock. They require no tools they literally just pop in. You don't need a dealer or mechanic to do that for you.
Here they are, in order of likelihood needed:
Small underbody clip, Toyota part number 90467-07201
Large underbody clip, Toyota part number 90467-09227
Mounting screw, Toyota part number 90159-60498
Go to page 8 and 9 of this document to see the mid underbody panel. The one that is hanging down on your car is the panel immediately in front of this, but it attaches the same way.https://www.dropbox.com/s/w5uy098ud63ue ... 4.pdf?dl=0