The reference to tapping is why I question if we are all talking about the same thing.
The square shape is because carriage bolts were used originally.
There are a number of ways to do this job. You can drill a round hole and file square for carriage bolts, just use a standard bolt in a round hole, or what I like t do and that is tack weld nuts on the top side so you run the bolts up from the bottom.
