i've used one from halfords in my 350z, cost about £8.99, it's now in my 370z, maybe a little bit big with nothing in it tho even folded in half, works fine and stops your stuff rolling about.
think it was this one - http://www.halfords.com/motoring/roof-bars-roof-boxes/roof-box-accessories/halfords-essentials-12-hook-luggage-net