There are plenty of curved cabinet builds out there.
I offer nothing new to the seasoned veteran, but hopefully a new DIYer will find something useful in this build.
Ignore the speaker cutout. This is just some scrap wood.
I knew I wanted a depth of 20", so I set a tape measure at the 20" line and used a ruler to check for a pleasing curve radius and offset. I found a 25" radius to be a good choice, with a 3" offset from the 10" center line. This meant that I can use a circle jig that I made in a previous speaker stand build
Again, I held my fingers at different positions along the ruler and swung it in an arc and looked for a curve that I liked.
This would look a little less confusing if I had used a large sheet of scrap under the workpiece, but I just used small scrap pieces, so I'll try to explain how I lined things up.
I measured over from the table edge to center the long scrap plywood piece, since I knew that my pivot center is 10" - 3" or 7" from the edge.
Once I had the long plywood piece centered, I held a board along the two table edges to center my workpiece, using small scrap pieces under the workpiece for stability during arc-routing.
Once everything was centered, I shot some short brads into the workpiece to secure it to the long plywood backer. I also secured a small scrap piece with a small pivot hole at the 25" mark, to the same backer.
With everything aligned and secured with brads, I could now move the entire assembly toward the table edge, so I could get some clamps on it.
Again, it would be less confusing if I had used a larger backer.
Here I've secured the circle jig to the router.
Secure the pivot.