You can create the bottom the same as the top to get it flat. Just have a horizontal section in the sketch with a radius (fillet) where the slope begins.
This is the option to change the orientation of the swept sketch:
This is not normally recommended for sweeping round features but the option is there and should be noted as to the difference. Normally, I could connect a datum curve to the sweep trajectory to make a clean tangent if I needed to do this with a separate feature. It is a bit of work, but the datum curve is good about joining 3D curves where and when needed.
Remember to use a coordinate system in the sketch to assure proper wrapping of the sketch.