This is made by Contico and can be found at WalMart and Lowes. $25 in AZ.

The great thing about this box is that the height is just below the tailgate. With the tailgate closed it looks like a perfect fit. I have mine up against the tailgate to make the lid easy to open. There is room between the box and seats to put more junk.

http://www.myplaneonline.com/element/images/storagebox2.jpg

Walmart also has a net style holder that's $12. A real booger to put together....just too simple I guess but it has three compartments and fits right across the rear of the Element. Just another idea.