[Mike]

You can create your own tongue and groove using a 1/4" straight cutting bit. Mark your material and plow out the 1/4" groove on the stiles. Next make rabbit or rebate cuts on the ends of the rails to fit your groove.

 

[Mike]

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.

 

[Mike]

I think it is time to stop trying to re-engineer the wheel. If you buy whichever hardwood is cheapest in your location and seal it with a good spar varnish it should last many years. If you want overkill then use epoxy paint. Either way will work fine.