3dobjects

HD = 6; /* hole diameter */
HYO = 9.5; /* hole Y offset (to center) */
W = 15; /* width */
D = 15; /* depth */
D1 = 5; /* depth thick section */
PT = 4; /* panel thickness */
WT = 2; /* washer thickness */
CR = 3; /* corner radius */

/* Create washer base: */
difference() {
    /* Create rectangle with two rounded corners using convex hull: */
    hull() {
        cube([W,D1,WT]);
        translate([0,D-CR,0]) {
            translate([CR,0,0]) cylinder(h=WT, r=CR, $fn=50);
            translate([W-CR,0,0]) cylinder(h=WT, r=CR, $fn=50);
        }
    }
    /* Remove round hole: */
    translate([W/2,HYO,-1]) cylinder(h=WT+2, d=HD, $fn=50);
    /* Remove slot: */
    translate([W/2,HYO+HD/2,WT/2]) cube([HD,D-HYO+1,WT+2], center=true);
}
/* Create thick section: */
translate([0,0,WT]) cube([W,D1,PT]);