Csdrhrt

Technical manuals from the 1960s and 1970s sometimes had eye-catching CAUTION boxes surrounded by wavy or curly lines.

My best attempt at re-creating this is not ideal as I can't get the snake to line up on all corners:

begin{tikzpicture}

    draw[very thick,decorate,

        decoration={snake,segment length=9.5pt}]

        (0,0) rectangle (140pt,40pt)

        node[pos=0.5] {textbf{CAUTION}};

end{tikzpicture}

I would also be willing to use any other package which can produce fancy boxes with borders.

I would employ Jake's complete sines here to modify your proposal to something that can also be used in a tcolorbox

documentclass[a4paper,12pt]{article}

usepackage[many]{tcolorbox}

usetikzlibrary{decorations}

begin{document}



pgfdeclaredecoration{complete sines}{initial}

{

    state{initial}[

        width=+0pt,

        next state=sine,

        persistent precomputation={pgfmathsetmacromatchinglength{

            pgfdecoratedinputsegmentlength / int(pgfdecoratedinputsegmentlength/pgfdecorationsegmentlength)}

            setlength{pgfdecorationsegmentlength}{matchinglength pt}

        }] {}

    state{sine}[width=pgfdecorationsegmentlength]{

        pgfpathsine{pgfpoint{0.25pgfdecorationsegmentlength}{0.5pgfdecorationsegmentamplitude}}

        pgfpathcosine{pgfpoint{0.25pgfdecorationsegmentlength}{-0.5pgfdecorationsegmentamplitude}}

        pgfpathsine{pgfpoint{0.25pgfdecorationsegmentlength}{-0.5pgfdecorationsegmentamplitude}}

        pgfpathcosine{pgfpoint{0.25pgfdecorationsegmentlength}{0.5pgfdecorationsegmentamplitude}}

}

    state{final}{}

}



tikzset{oldfashioned sines/.style={very thick,line cap=rect,% <- thanks to quark67!

     decorate,decoration={complete sines,segment length=#1,path has corners}},

oldfashioned sines/.default=9.5pt}



newtcolorbox{Caution}{%

    width=140pt,height=40pt,

    colback=white,halign=flush center,valign=center,

    enhanced,% jigsaw, breakable, % allow page breaks

    frame hidden, % hide the default frame

    overlay={%

     draw[oldfashioned sines]

        (frame.south west) rectangle (frame.north east);

     draw[oldfashioned sines]

        ([xshift=1pt,yshift=0.4pt]frame.south west) rectangle ([xshift=1pt,yshift=0.4pt]frame.north east);  

    },

    % paragraph skips obeyed within tcolorbox

    parbox=false,

}



begin{Caution}

textbf{Caution}

end{Caution}



begin{tikzpicture}

    draw[oldfashioned sines]

        (0,0) rectangle (140pt,40pt)

        node[pos=0.5] {textbf{CAUTION}};

end{tikzpicture}



begin{tikzpicture}

    draw[oldfashioned sines]

        (0,0) rectangle (140pt,40pt)

        node[pos=0.5] {textbf{CAUTION}};

    draw[oldfashioned sines]

        (1pt,0.4pt) rectangle (141pt,40.4pt)    ;

end{tikzpicture}

end{document}

I'd like to thank quark67 for the great suggestion to add line cap=rect.

ADDENDUM: An attempt to address your request.

documentclass[a4paper,12pt]{article}

usepackage[many]{tcolorbox}

usetikzlibrary{decorations}

begin{document}



pgfdeclaredecoration{complete sines}{initial}

{

    state{initial}[

        width=+0pt,

        next state=sine,

        persistent precomputation={pgfmathsetmacromatchinglength{

            pgfdecoratedinputsegmentlength / int(pgfdecoratedinputsegmentlength/pgfdecorationsegmentlength)}

            setlength{pgfdecorationsegmentlength}{matchinglength pt}

        }] {}

    state{sine}[width=pgfdecorationsegmentlength]{

        pgfpathsine{pgfpoint{0.25pgfdecorationsegmentlength}{0.5pgfdecorationsegmentamplitude}}

        pgfpathcosine{pgfpoint{0.25pgfdecorationsegmentlength}{-0.5pgfdecorationsegmentamplitude}}

        pgfpathsine{pgfpoint{0.25pgfdecorationsegmentlength}{-0.5pgfdecorationsegmentamplitude}}

        pgfpathcosine{pgfpoint{0.25pgfdecorationsegmentlength}{0.5pgfdecorationsegmentamplitude}}

}

    state{final}{}

}



tikzset{oldfashioned sines/.style={very thick,line cap=rect,% <- thanks to quark67!

     decorate,decoration={complete sines,segment length=#1,path has corners}},

oldfashioned sines/.default=9.5pt}

defLstAnchors{"south west","south east","north east","north west"}

newtcolorbox{Caution}{%

    width=140pt,height=40pt,

    colback=white,halign=flush center,valign=center,

    enhanced,% jigsaw, breakable, % allow page breaks

    frame hidden, % hide the default frame

    overlay={%

     foreach XX  [count=YY,remember=XX as LastXX (initially north west)]

     in {south west,south east,north east,north west}

     {ifnumYY<3

     draw[oldfashioned sines] (frame.LastXX) -- (frame.XX);

     draw[oldfashioned sines] ([xshift=1pt,yshift=0.4pt]frame.LastXX) -- ([xshift=1pt,yshift=0.4pt]frame.XX);

     else

     draw[oldfashioned sines] (frame.XX) -- (frame.LastXX);

     draw[oldfashioned sines] ([xshift=1pt,yshift=0.4pt]frame.XX) -- ([xshift=1pt,yshift=0.4pt]frame.LastXX);

     fi}

    },

    % paragraph skips obeyed within tcolorbox

    parbox=false,

}



begin{Caution}

textbf{Caution}

end{Caution}

end{document}