$fn       = 100;
hr        = 2;
diatop    = 80;
rim       = 20;
diabottom = diatop - rim;
hlen       = 50;

  difference() {
    translate([0,0,0]) 
    cylinder (h=hlen, d2=diatop, d1=diabottom, center=false); 
    translate([0,0,2]) 
    cylinder (h=hlen, d2=diatop-4, d1=diabottom-4, center=false); 
    difference() { 
     translate([0,0,hlen-3.5]) cylinder (d=diatop+4,h=hr , center=false); 
     translate([0,0,hlen-5.5]) cylinder (d=diatop-2, h=hr+4  , center=false); 
      }
    }