I don't have 3rd row either, so not sure exactly how it looks, I do know that the sliding cargo tray models make the trunk flush with the seats when folded down, so you don't need to add anything.
I would do something like this if you want to add a platform:
5th Gen 4Runner Sleeping Platform Build - YouTube
I had zero problems blowing up a twin air mattress and sleeping on that, without changing anything, the hump from trunk to folded seats wasn't felt at all.