Pi Spoiler...
I've got a Pi Spoiler for mine, although I still haven't got it fitted despite the fact its been laying under my bed for nearly a year!!
I think Ben B has one fitted on his...its basically the same as the MS Designs one and fits your description exactly....a little bit bigger than the standard one but not as big as the Abarth.
(And it screws on so you'll need to drill three holes in your boot!!)