Realize that any site that gets oodles of sun are going to dry out a hanging basket in a heart beat. If I were going to try any clematis in a hanging basket, it would be a variety that stays very small such as integrifolia the species or fremontii or something similar and I would get as large a basket as possible. Clematis like a steady supply of moisture and trying to maintain that in a hanging basket in the heat of the summer is going to be difficult. Why not try growing them in large pots instead of a hanging basket? The types suited to a pot are going to be way more numerous than what would be suitable for a hanging basket.