stereo install: missing piece?

I purchased this single DIN kit for my 2006 NF:
Metra 99-7320 In-dash Receiver Kit Fits select 2006-08 Hyundai Sonata models — single-DIN radios at Crutchfield.com

But now that I've installed it, I see that there is no way to fasten my stereo to it. The stereo just floats in the empty space. I found the fine print, "most vehicles will need extra installation parts." Some further clicking reveals that "A receiver mounting kit is needed to install an aftermarket receiver."

So... what is that? Where can I find it? Comparable double-DIN kits appear to have stereo mounting areas, whereas my single-DIN kit does not.

Thanks!
Mike