if you are worried about twisting ext from other aspects then when you do your layup you are going to have to orientate your fibres in more than 2 directions to stop this which will mean multiple layers in places. i think you will find a carbon skin for the hole wing will be over kill and just add wait. try to only use carbon were you need the strength ie in the spar and in the dbox possibly. you can purchase carbon T section that will provide you with more then enuff strength for your needs and only way a few grams and then its just a matter of crating the wing shape as light as posable so using a lighter weight material like mylar were you are just need to cover the structure. There was a program by guy martin not long ago about building a human powered aircraft and they used the same foam mylar technique

Hot wire cutting foam can be as easy as a length of stainless steel wire and a 12v battery and a bow of some kind and then just two templates at ether end of the shape you want you also can get a range of densities of eps ranging from just under 1lb cubic foot to 3 or 5 so lots to choose from.