This is a design based on general deck rail mounts available. I didn't like how most of them rely primarily on the plastic mount to hold up the rail. Ussually they have one set of screws entering perpindicular to the 2X4 and the second set on an angle but not far enough into the wood in my opinion. This could be to save injection mold costs which aren't a concern for printing. So I made this a little taller and changed the angle to hit farther into the 2X4 on all 4 screws. Screws are offset to avoid hitting each other or weakening the wood.

Use #8 Deck Screws. I used 10's becasue that was all I had and didn't want to spend another$10. So they're a little tight. Also, do not use a driver to fnish seating or you will most likely split the plastic.