You can make your own car trash bag by sewing pieces of fabric together. You can use canvas, twill, outdoor fabric, or a combination of materials. It’s a long process, and you will need a bit of crafting and sewing experience.
In general, you cut your chosen material to size, sew the pieces together inside out, and then turn the bag so the lining is inside. Attach the straps to the bag and hang it up in your car.