Better to mix chunks of polystyrene, or polystyrene packing "peanuts" (or "s" shapes) through the planting mixture in the lower part of the troughs - that way you avoid the problem of a 'perched water table' which would interfere with the proper drainage of the trough and still reduce the weight and amount of mix required to fill the area. Be sure to pile the filling high - it will surely sink a lot and spoil the finished effect - and, as David says, some rock work will add to the aesthetic effect too.
