Hi Chris,
Actually, the walls are parallel, it's just a trick of the eye they way the blade is terminated on the ends!
If you cross section at centreline, and extract that curve, and use it to drive the cross section of the wrapped pocket, then you can finish that geometry complete in a single feature. When you set a cross section curve, FC then gives you the same wall/floor options that you get if you have a draft angle set, so you can choose radius mills or whatever you like to fully machine the profile, all wrapped around the axis.
For some reason I am not able to upload pictures today, but you can see here https://imgur.com/hDO0Cu4