Gfyuki

Technical manuals from the 60s and 70s 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.

To very first approximation I would employ Jake's complete sines here to modify your proposal to

documentclass[a4paper,12pt]{article}

usepackage{tikz}

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}{}

}



begin{tikzpicture}

    draw[very thick,decorate,

        decoration={complete sines,segment length=9.5pt}]

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

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

end{tikzpicture}



end{document}

There are many conceivable ways to modify this, e.g.

begin{tikzpicture}

    draw[very thick,decorate,

        decoration={complete sines,segment length=9.5pt}]

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

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

    draw[very thick,decorate,

        decoration={complete sines,segment length=9.5pt}]

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

end{tikzpicture}

along with more fancy options.