How to make open ended filled shapes?

[Byron Stevens]

I'm trying to make a rounded rectangle shape that has an open-ended  
stroke on one side. Thought I had done this a long time ago, but but  
can't seem to figure it out now :-)

Basically I'm trying to make a selected tab look - rounded rect with  
square bottom (sort of a square D shape), but the bottom straight line  
should have no stroke (or appear that way). I still need to fill the  
space though. So far any attempt to join shapes results in a complete  
stroke all round.
I see some shapes in my shapes pane that have this feature (arcs, for  
example), so it seems to be possible...

Hopefully I didn't miss something obvious - anyone have a clue how to  
do this?
Thanks!

Byron