Looking for a pattern to make plastic bag holder out of materail?

Answer 1

I've done this. Just take a piece of cloth, fold the top edge over to the back and sew it down (leave enough room for inserting elastic later).
Fold the bottom edge over to the back and sew it down (leave room to insert elastic).
Fold the whole piece in half with the back outward. Sew it up one side. Now you have a tube.
Insert the elastic and draw it up to gather the top and bottom of the tube.
Attach a loop at the top to hang it.

Answer 2

Get a tea towel and thread elastic thru top and bottom with the side of the tea towel sewn up. Attach a length of cord from which you can hang the creation.