Arsthbt

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.

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
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


tikzsetoldfashioned 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

newtcolorboxCaution%
 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,


beginCaution
textbfCaution
endCaution

begintikzpicture
 draw[oldfashioned sines]
 (0,0) rectangle (140pt,40pt)
 node[pos=0.5] textbfCAUTION;
endtikzpicture

begintikzpicture
 draw[oldfashioned sines]
 (0,0) rectangle (140pt,40pt)
 node[pos=0.5] textbfCAUTION;
 draw[oldfashioned sines]
 (1pt,0.4pt) rectangle (141pt,40.4pt) ;
endtikzpicture
enddocument

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