x_size = 30;           // horizontal outer size of the aquarium pedestal. 
y_size = 30;           // 
thickness = 10;         // thickness of the pad bellow aquarium. hairs lenght is 12mm. 
rim_height = 10;        // height of upper rim for fixing the aquarium in position.

difference () {
    rotate([0,0,45])
            cube([x_size, y_size ,thickness]);

//    cube([x_size - 2* thickness, y_size - 2* thickness, pedestal_height - rim_height - thickness]);
}

            translate ([0, 0, thickness])
                    cube([sqrt(pow(x_size,2) + pow(x_size,2))/2, sqrt(pow(x_size,2) + pow(x_size,2))/2, rim_height]);