Self-made image, converted from the following vector PostScript source code:

%!
2 3 div dup scale 1 setgray 18 setlinewidth/X{.5 add 72 mul
exch .5 add 72 mul exch}def/l{X lineto}def 1 setlinejoin
0 7 X moveto 8 7 l 8 8 l 7 8 l 7 0 l 6 0 l 6 8 l 5 8 l 5 0 l
4 0 l 4 8 l 3 8 l 3 0 l 2 0 l 2 8 l 1 8 l 1 0 l 0 0 l 0 1 l 8
1 l 8 2 l 0 2 l 0 3 l 8 3 l 8 4 l 0 4 l 0 5 l 8 5 l 8 6 l 0 6
l 0 7 l closepath gsave 0 setgray 36 setlinewidth stroke
grestore stroke/r{72 mul exch 72 mul exch rlineto}def
36 setlinewidth 0 setgray
1 2 7{/z exch def 0.5 2 7.5{z X moveto 1 0 r stroke}for}for
2 2 6{/z exch def 1.5 2 5.5{z X moveto 1 0 r stroke}for}for
1.5 2 6.5{/z exch def 1 2 7{z X moveto 0 1 r stroke}for}for
0.5 2 7.5{/z exch def 2 2 6{z X moveto 0 1 r stroke}for}for
1 setgray 18 setlinewidth
1 2 7{/z exch def 0.48 2 7.48{z X moveto 1.04 0 r stroke}for}for
2 2 6{/z exch def 1.48 2 5.48{z X moveto 1.04 0 r stroke}for}for
1.48 2 6.48{/z exch def 1 2 7{z X moveto 0 1.04 r stroke}for}for
0.48 2 7.48{/z exch def 2 2 6{z X moveto 0 1.04 r stroke}for}for
showpage
%EOF