The customer doesn't hafta buy it. We're talking $250 or $300. Why not buy a router ?? It'll come in handy for so many things. Be a great investment, regardless of who buys it.
His method is making an outside cut, then, an inside cut and a few more and then you chisel the middle out. That is silly, unless you just bought a saw yesterday and wanna find things ya never wanna do again. Ya might as well use your teeth or buy a beaver.
And no, you are not making a mortise joint. This is not furniture 101. He's making a 48" rout down a treated wooden post. Your method will eventually work, but you'll need half a morning, and then it still won't be right. Either a dado or router are the only real ways to do this. Even a CNC machine would be difficult.Get out of the 1800s and step into the 21st century, sparky.