Make new ones out of acrylic, polyboard or wood. Mahogany or teak are best for exterior wood. Oak, ash and woods that have open grain are much more prone to get mold and mildew. If you use polyboard, be sure that it is stabilized for exterior use. Both polyboard and acrylic are very heavy.
The boards need an angle to prevent water intrusion when it rains or water drips on them. Just take a look at someone elses boards and you can figure it out.
