Try this as a starting point. You should be able to introduce some randomization and rounded corners.

def a_tiless (expr p) =

T:=identity;

pickup PenC;

path q, qq; q = bbox p;

picture tmp_pic;

uu := max(u, (xpart urcorner q - xpart llcorner q)/100, (ypart urcorner q - ypart llcorner q)/100);

tmp_pic := image(

for i = xpart llcorner q step 2uu until xpart urcorner q:

for j = ypart llcorner q step 2uu until ypart urcorner q:

qq := punked (((-.5uu,-.5uu)--(.5uu,-.5uu)--(.5uu,.5uu)--(-.5uu,.5uu)--cycle)

)

shifted ((.6i,.6j) );

if xpart (p intersectiontimes qq) < 0:

thclean qq;

thdraw qq;

fi;

endfor;

);

clip tmp_pic to p;

drawoptions();

draw tmp_pic;

enddef;