OEM spoiler hollow?

Where are you going that you need additional downward force to keep your Prius on the pavement?

 

Then ask a body shop to do it for you. That way it will not look DIY.

 

Better look at it real carefully it is hollow very thin and the crap breaks like you wouldn't believe so now the new spoiler that you're going to stick on top of the old spoiler does it have little stanchions on it so it can stand up on top of the old spoiler like little feet and I'm sure they're plastic you may be wanting to look at a two-step epoxy to glue the upper new spoiler stanchions right down on the year existing spoiler that's been cleaned well marked and centered then epoxy what you just marked down to the old spoiler a just as needed put two bricks on each side even if they're cinder blocks just to hold it down for a couple hours while it dries you use the right glue. It will take quite a bit to get it to lift off if you will and given the aim and everything you're going to get you're probably not going to get that much air under it anyway remember it's mostly for looks right

 

So are truck nutz, but yes, to answer your question the OEM unit is hollow.

Even though a Prius is not really capable of generating downforce with a fake rear airfoil you WILL get some wind loading and the OEM spoiler is fairly thin.
If nobody talks you out of the Wingo look, consider spraying a little expanding foam insulation maybe from an additional small hole aside the one you put the mounting bolts through to stiffen things up a bit.
That might keep the new wing (or wings) from taking flight when you're driving frugally and efficiently, trying to save the planet.

Good Luck!

 

I’ve taken the grapevine to Southern California and drooled over thought of having one installed. I was going downhill at 80 mph.