multiput and linethickness

\setlength{\unitlength}{2mm}
\begin{picture}(30, 20)
  \linethickness{0.075mm}
  \multiput(0, 0)(1, 0){31}{\line(0, 1){20}}
  \multiput(0, 0)(0, 1){21}{\line(1, 0){30}}
  \linethickness{0.15mm}
  \multiput(0, 0)(5, 0){7}{\line(0, 1){20}}
  \multiput(0, 0)(0, 5){5}{\line(1, 0){30}}
  \linethickness{0.3mm}
  \multiput(5, 0)(10, 0){3}{\line(0, 1){20}}
  \multiput(0, 5)(0, 10){2}{\line(1, 0){30}}  
\end{picture}