Box Extrusion to match surface - help

I am very new to Grasshopper and am attempting my first experiment.  I want to extrude a rectangular shape so that the ends along one side match the curvature of a previously defined surface.  I came close, but it is not perfect and the process is most likely very inefficient.  I am still having trouble understanding data flow and data matching, which I think is causing most of my problems with this one.  Any suggestions would be very much appreciated.  Thanks