How to make a square topped, round rect bottomed graphic?

[David Bovill]

What is the most efficient way to make this? I thought I could have a
polygon graphic which was opaque but not joined up and use that to add
square corners to a roundrect graphic background - but it seems that every
time I make it opaque it automatically joins the graphic points.

What I want to be able to do is have a vector graphic which has the option
to have rounded corners on either or both the top and bottom, whcih I can
script. The most efficient way I can thin of doing it at the moment is to
have a background roundrect graphic, on top of which I can place a polygon
line to create square corners, and a white opaque graphic just inside the
square boundary to cover up the round corners?

Is there a better way?