a thin sheet of alluminum and some braces won't cost that much... thin aluminum can be bend to straight edges over a sharp table edge fro example.... since you're not building it for offroading the aluminum can really be quite thin... if you fold up all the edges at a 90 degree angle the resulting shape is far less floppy than the original sheet, and when attached to existing or fabricated supports it will really work well.
mine cost around 12 € if i remenber correct
mine can be seen here
http://ecomodder.com/forum/showthrea...html#post35193
i've also made a small simulation of the hatch spoiler,... the result is see saw pattern of alternating vortices, the spoiler below the window seems to prevent the upwashing vortex from rolling over the window. this could prevent the deposit of dirt etc... in this case the bottom of the spoiler seems more important than the top, wich might explain the shape of the ax spoiler
the pink area is a residue of the big pink vortex... so likely it won't help FE, but if won't hurt it eider and it will keep the window clean