Had another thought that it is perhaps the cover from a relay box or a distribution panel? There are quite often oodles of bits of electrickery in undercarriage bays (so you would have Port and Starboard panels) and I am sure I have seen them made of composite type materials. I always maintained leckies could never find their own backsides with both hands so they identified all their boxes hence the number 4

I am going to discount anything engine bay as the heat would preclude using GRP etc.