I use one I kept from my previously owned 2001 Cadillac Catera. It is rectangular in shape and has plastic clips at each corner that clip rather well to the metal anchors. It is a little bigger than what I would like but the price was right. Maybe a quick measure for dimension between hooks and try a walk thru the local auto wrecking lots and check trunks. I would have to guess that you could find one near the size that would fit. Who knows they may only be a couple of bucks...............
07 e92 328xi/Space Gray/Black Dakota /Steptronic/Cold Weather/Premium/Sport/Nav/Xenon/ Style 185's
06 e90 330i/Alpine White/Terra Dakota /Steptronic/Cold Weather/Premium/Sport/CA/PDC/Xenon/Logic 7/Style 162's
06 e90 325i/Jet Black/Black Leatherette/Steptronic/Cold Weather/Style 197's