1 cubic foot = 1,728cubic inches = 7.4805gallons If it's full of water, it weighs about 62pounds 6.8ounces. If it's full of gold, it weighs about 1,204pounds 13.7ounces.
Obtain any material that can be carved, something to carve it with, and a ruler. Carve from the material a cube whose edge length is a foot. You now have a cubic foot of the material. Fashion a knife...