You probably could! Id be inclined to measure out to the hatch on either side, whatever that no. Is, looks to me abut 100 - 150mm then what I would do is make a mould top and bottom of the centre of the wing so it makes 2 half's, use them as moulds, (did I say it twice? Yep it's staying in there lol ) then I'd cut the blade and extend it from the centre! Maybe make some fibre glass supports inside the blade at your joins just to be safe!
I'm not a fibreglasser but it's just my 2 penneth! If it's too much hassle then maybe have a chat with edrx7, he does spoilers and all sorts! Maybe he could point you in the right direction! Personally though I'd just get stuck in, spesh if the wing was cheap ?