I would prefer to dig out the wheel wells after applying the balsa skins, but I would use vacuum to apply them so if you just use the off-cuts and weight I guess you have a choice. I would worry that the balsa skins would "sag" and not follow the airfoil as good just above the wells. About a spar I would make it for sure, properly with the spar caps attached to a shear web similar to what you would find in a built up or molded wing, but perhaps you can find some examples where others have done what you want (quicker and simpler) with success. I always like a proper aircraft structure and in that context a wing spar seems to me a vital part of it, but I guess it's not always necessary.