A big bag xTrash Bag, Zumiez bag, etc.)
A big piece of wood
A piece of metal that can support the piece of wood like a stop sign
Ok, first take the bag you haave and fill it with sand
Then Put the metal piece on top of the bag, then the piece of wood on top of the metal.
Sometimes this doesn''t work, you need to have the right metal and wood for it. Let me know if it works for you.