Yhyjyk

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:

begintikzpicture
 draw[very thick,decorate,
 decoration=snake,segment length=9.5pt]
 (0,0) rectangle (140pt,40pt)
 node[pos=0.5] textbfCAUTION;
endtikzpicture

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
usepackagetikz
usetikzlibrarydecorations
begindocument

pgfdeclaredecorationcomplete sinesinitial

 stateinitial[
 width=+0pt,
 next state=sine,
 persistent precomputation=pgfmathsetmacromatchinglength
 pgfdecoratedinputsegmentlength / int(pgfdecoratedinputsegmentlength/pgfdecorationsegmentlength)
 setlengthpgfdecorationsegmentlengthmatchinglength pt
 ] 
 statesine[width=pgfdecorationsegmentlength]
 pgfpathsinepgfpoint0.25pgfdecorationsegmentlength0.5pgfdecorationsegmentamplitude
 pgfpathcosinepgfpoint0.25pgfdecorationsegmentlength-0.5pgfdecorationsegmentamplitude
 pgfpathsinepgfpoint0.25pgfdecorationsegmentlength-0.5pgfdecorationsegmentamplitude
 pgfpathcosinepgfpoint0.25pgfdecorationsegmentlength0.5pgfdecorationsegmentamplitude

 statefinal


begintikzpicture
 draw[very thick,decorate,
 decoration=complete sines,segment length=9.5pt]
 (0,0) rectangle (140pt,40pt)
 node[pos=0.5] textbfCAUTION;
endtikzpicture

enddocument

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

begintikzpicture
 draw[very thick,decorate,
 decoration=complete sines,segment length=9.5pt]
 (0,0) rectangle (140pt,40pt)
 node[pos=0.5] textbfCAUTION;
 draw[very thick,decorate,
 decoration=complete sines,segment length=9.5pt]
 (1pt,0.4pt) rectangle (141pt,40.4pt) ;
endtikzpicture

along with more fancy options.