Diameter = 101.6; // 0.1
Depth = 6; // 0.1

Inset = false;
Inset_depth = 4; // 0.1
Inset_width = 5; // 0.1

$fn = 180; // 0.1

rotate([0, 180, 0]) {
difference() {
cylinder(h = Depth, d = Diameter); 
if (Inset) {
    cylinder(h = Inset_depth, d = Diameter - 2*Inset_width);
};
};
};