Otdfbt

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:

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.

ADDENDUM: An attempt to address your request. It comes with a "new" decoration symmetric sines that differ from Jake's complete sines in that the number of oscillations is half-integer. Therefore the result is symmetric under reflections about an axis that runs through the middle of the path.

documentclass[a4paper,12pt]article
usepackage[many]tcolorbox
usetikzlibrarydecorations
begindocument

pgfdeclaredecorationsymmetric sinesinitial

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

 statefinal[width=pgfdecorationsegmentlength]
 pgfpathsinepgfpoint0.25pgfdecorationsegmentlength0.5pgfdecorationsegmentamplitude
 pgfpathcosinepgfpoint0.25pgfdecorationsegmentlength-0.5pgfdecorationsegmentamplitude
 


tikzsetoldfashioned sines/.style=very thick,line cap=rect,% <- thanks to quark67!
 decorate,decoration=symmetric sines,segment length=#1,path has corners,
oldfashioned sines/.default=9.5pt
defLstAnchors"south west","south east","north east","north west"
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=%
 foreach XX [count=YY,remember=XX as LastXX (initially north west)]
 in south west,south east,north east,north west
 ifoddYY
 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,


beginCaution
textbfCAUTION
endCaution
enddocument