I have used 2"x3" pvc coated metal garden fencing and landscaping fabric. You will need a frame of some sort, be it wood, pvc, wood or whatever. Once the frame is built attach the pvc coated fencing and line the fencing with the fabric. You can affix the fabric with zip ties, something else or nothing at all.. The medium will hold the fabric up against the fencing as you fill as you fill it in.
You really don't want to use the fabric without using something like the fencing to back it up. The fabric can tear.