Plastic Rivits

Get the proper clips at your dealer, they are only a couple dollars each.

 

For the main under panel the parts numbers are in the attached. For all of them, have a look at the official Toyota USA website:


Toyota Parts and Accessories - Official Online Store


It takes a bit of rooting around; they're not all in one place.

I've got a few tips for preserving them, in the oil change link in my signature.

 

Toyota uses 7mm plastic rivets.

These work perfectly and cost much less than those from the Toyota dealer.

Ziruiautopart 100 Pieces Pin in 7mm Hole Compatible for Toyota 9046710183 Plastic Rivets Car Clips Fastener Push Clips Retainer for Cars for Scion Lexus

 

True in most spots, but a few of them are 9mm size, including one on the oil change flap. I bought a box of 7mm ones through Amazon, much less expensive than dealer prices. They function about equally as (un)well as the original ones.
The biggest irritation with buying them from Amazon is that half their listings for such fasteners fail to specify the fastener diameter---although they may show some scarcely relevant dimension, such as the size of the box they're packed in.

 

Two I think. The ones with the grey centre piece?

 

On the flap itself? Perhaps you're thinking of taking the entire underpanel off?

 

Could well be; I take the full panel off at oil changes, so not that conscious of the flap zone.

 

Instead of plastic rivets, I used M6 rivet nuts that fit the large 9mm holes perfectly without drilling and M6 stainless steel bolts with large neoprene fender washers, They do not shear off like the plastic rivets.

 

In my experience at least, the plastic fasteners fail due to broken jaws and or snapped center caps, due to the mechanism jamming with grit. Rushed “professional” service doesn’t help. See oil change link in my signature for preservation tips.

 

Yep, I hate breaking those plastic clips...and there's quite a few different sizes on our Toyotas! I have two big plastic containers with assorted sizes and, so far, have been able to find suitable replacements. And, of course, small zip-ties can be a last-ditch savior if the piece will let you get one in there. (Often, you can't)
The worst clips are those ones on the little doors under the rear seat that cover the air filter for the battery cooler fan....they break about 95% of the time.

 

Zip ties, and maybe heavy gauge copper grounding wire, are good items to keep in a "car first aid" kit. The copper wire can serve as heavy-duty twist ties, when something's dragging or whatever.