Wanart, Biscuits are fine for furniture indoors but a poor choice for outside use. In order to keep things simple you can build a simple half lap joint on each corner. Use a straight bit to remove half the material from each piece, place the two pieces together and you have a perfect match. You would want to use either Titebond 3 or a polyurethane glue for this project. Gorrila glue is one popular brand name. Be sure to measure the long diagonals so you are square when you clamp this up. Once your glue dries you will want to install dowels through the joint to keep it pinned in place. (Minimum of 3, 4 preferred) Be sure to use hardwood dowels such as oak, cherry or walnut. You may want to use a contrasting color rather than trying to make them match your composite. I would seek advice on the finish from a local building supply company. They will steer you to the products that will work best in your area.