I added a car organizer to the "right side" of my trunk for maps and bike registration, and I can still put both helmets in the trunk.
I used velcro to hold it in, and it works fine for me.
Since then, I purchased a Utopia back rest, and the back rest has the pouch on the back of it.
Now I just put my maps in it, and the trunk organizer has a couple of ink pens, bike registration, and a package of "handi-wipes" in it...