Measure the height of the smartphone and cut the plexiglass to a height that is 1" higher. This will allow you to overlap the plexiglas by 1/2" to the top and bottom pieces of wood.
Attach the front and back plexiglass to top and bottom wood pieces using the screws. Be sure to size the opening using the smartphone so that it slides smoothly into the routed slots.
To ensure the enclosure is waterproof, use some silicon between the wood and the plexiglass.