+\startuniqueMPgraphic{box}
+path a, b;
+% Create a rectangle of the full size
+a := unitsquare xyscaled(\overlaywidth,\overlayheight);
+% Randomize it a bit
+b := a randomized (\overlaywidth / 25) ;
+% And draw it
+drawoptions (withpen pencircle scaled .75mm withcolor black) ;
+draw b;
+\stopuniqueMPgraphic
+
+\defineoverlay[box][\uniqueMPgraphic{box}]
+
+% Define \{start,stop}boxed with a nice metapost box around it.
+\defineframedtext[boxed][background=box,frame=off]
+% A boxed variant with the width forced to page width. This is used in
+% \startrans, since the content somehow doesn't scale the box around it
+% properly.
+\defineframedtext[wideboxed][background=box,frame=off,width=\textwidth,offset=.02\textwidth]
+