In the Top View create a box 35 units long, 35 wide and -445 high. Place it at about 37.5 units from the left and bottom edges of the frozen object. Convert the box to an editable poly.
Select the bottom polygon and move it (in the Top View) about 22.5 units towards the corner of the frozen object. Then Outline this polygon -2.0 units.
Exit editing mode and add a Symmetry modifier to the object. Go to the Mirror subobject by expanding the Symmetry modifier (a). Move the Mirror to 0 on the X axis (in the Top View) (b). You need to check the Flip box in order to make the symmetric objects visible (c).
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