Signicaid Sandwich Frames are the best we've found. Made from heavy-duty plastic in different color options, they will last for years out in the elements. They lay flat for storage,  can be filled with water or sand to keep them standing in heavy winds, & have easy change options available including: reader board inserts, white board inserts, & chalk board inserts. There are many sizes to choose from-- we really love them.

This truly is a great price, so great we are purchasing one for our company-- same price, same link, as we are offering you.