Use a child's wooden spinning top as a plumb bob. Suspend it point down on a string; add weight by dropping a few metal washers over the string.
Or make a plumb bob from an empty tin. Mark dead centre on the bottom; puncture it and insert a nail through the hole as your marker. Poke two holes exactly opposite each other in the top of the tin, just under the rim, and form a short yoke with a piece of string. Fill the tin with sand or gravel and suspend it from a string.