Revolve (or Sweep?) past your end points, then cut or trim the end profile away
Do you have a specific reason to be using surfaces? May I recommend to you working more in solids.
Using solids saves having to create extra features as surfaces to do the same thing solids will do.
I'm a big advocate for 'Hybrid' modeling, using surfaces as construction tools to build solids.
I found a planar surface on the base ends between the end of the surface and the semi circular sketch. Then a boundary surface between the end of the planar surface and the current curved upper surface worked a treat. Don't forget to set the tangency as appropriate.
Hope this helps.
thanks everyone! i was able to get it after the advice i received