[pstricks] problem with sharp corners

[Goebel, Juergen, OPE26]

Hi,

please have a look at the enclosed example file. The corners 
of pseudo-3D-structure are don't fit to the neighboring lines,
as you can see. This is for sure not the high-LaTeX-quality,
we're used to. So, what do I have to do to enhance this
picture? (I tried it with arrows=cc-cc and the like but I didn't
get through.)

Thanks in advance,

Jürgen



\documentclass{scrartcl}
\usepackage{pstricks}
\begin{document}
 \psset{unit=1mm}
 \begin{pspicture}(0,0)(100,50)
  \psset{linewidth=0.4mm}
  \rput(20,40){%
     \pspolygon(-14,9)(14,9)(14,-9)(-14,-9)
 
\pspolygon[fillstyle=solid,fillcolor=lightgray](8,-4)(-8,-4)(30,-34)(80,-34)
 
\pspolygon[fillstyle=solid,fillcolor=lightgray](8,4)(8,-4)(80,-34)(80,-8)
     \pspolygon[fillstyle=solid,fillcolor=white](-8,4)(-8,-4)(30,-34)(30,-8)
     \pspolygon[fillstyle=solid,fillcolor=white](-8,4)(8,4)(80,-8)(30,-8)
  }
 \end{pspicture}
\end{document}