Perhaps you mean move it up? Like:

$fn=50;
r=7.5;
ra=4;
translate([0,0,10])
difference(){
union(){
cube([50,30,20],true);
translate([-25,0,0]) color("Magenta") cylinder(20,15,15,true);
translate([25,0,0]) color("Magenta") cylinder(20,15,15,true);
}
translate([15,0,ra]) color("red") cylinder(20,r,r,true);
//translate([15,0,ra]) color("red") minkowski()
{cylinder(20-2*ra,r-ra,r-ra,true); sphere(r=ra);}
translate([-15,0,10]) rotate([90,0,0])  color("gray")
cylinder(30+ra,r,r,true);
for(y=[-15:5:15]) {translate([-15,y,10])  slot();}
}

module slot(){rotate([90,0,0])  color("blue") cylinder(4,r+ra,r+ra,true);}

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I had thought from the description that the OP might want to hollow out the
underside, similar to that of the Lego brick.

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