Sounds like filter paper to me (usually a circle of the paper).
it is called a net
The net of a 2 dimensional shape can be folded into a 3 dimensional shape
The net of a 3D shape
When a circle is folded along its diameter, it creates two congruent halves or semicircles.
If I understand your question, the shape would be "geometrical"
It is a glass funnel into which can be placed a circle of folded filter paper (so that it too is the shape of the funnel) liquids may then be poured into the funnel and while the liquids will drain through (to be collected at the base of the funnel), any solids will be retained on the paper.
Funnel paper is typically used for filtering liquids or powders in laboratories or during chemical experiments. The paper is designed to effectively separate solids from liquids by allowing the liquid to pass through while trapping the solid particles. This process is commonly used in procedures such as filtration and chromatography.
A filter funnel is a funnel-shaped glass or plastic laboratory equipment used to separate liquids from solids or to filter particles from liquids. It is typically used in conjunction with filter paper to collect the solid particles while allowing the liquid to pass through. The liquid is poured into the funnel with the filter paper in place, and gravity or vacuum filtration is used to separate the components.
A tornado is called a funnel because of its shape - it often appears as a rotating column of air that narrows towards the base, resembling a funnel or a cone. This funnel shape is created as the tornado draws in debris and moisture from the ground, giving it its characteristic appearance.
To prepare a filter paper for filtration, first fold the filter paper in half and then in quarters. Next, open the filter paper to form a cone shape and place it in the filter funnel. Wet the filter paper with a small amount of the solvent being used for filtration to help it adhere to the funnel.
tornado clouds
it is called a net
A violent windstorm that often takes the shape of a funnel is called a tornado.
Funnel-shaped
If it is folded out it is called a net. It forms two circles and a rectangle.
A funnel shape
cone