By saying "tapered" is the channel itself squared off so to speak...straight sides...flat bottom? The channel you have looks straight, from one surface to a couple of inches deep. You didn't state the size or properties of the channel.
But, from looking at it, you might make a jig to hold it through the table saw, by having one end down and propping the other end up, and making passes through the blade. IOW, the channel is flat, but on an angle in the length of the wood.
I had a similar project on one end of my concrete patio next to the house. In a heavy rain, water would puddle there. So, I made a ramp for my skill saw to start with the blade skimming the patio, and tipping down as the saw is pushed towards the edge of the patio. It worked great.
.
But, from looking at it, you might make a jig to hold it through the table saw, by having one end down and propping the other end up, and making passes through the blade. IOW, the channel is flat, but on an angle in the length of the wood.
I had a similar project on one end of my concrete patio next to the house. In a heavy rain, water would puddle there. So, I made a ramp for my skill saw to start with the blade skimming the patio, and tipping down as the saw is pushed towards the edge of the patio. It worked great.


.