Old railroad line. If you can locate some, those who have it usually have a way to cut it, aka, select your size.
I have an old blue 10' 3pt disc that I constructed a wooden deck(box) on the top w/sides that covered the entire disc. I filled that with about 30-40 cinder blocks that are held in place by the box walls, with weight spread evenly over disc. Didn't know if the tractor would pick it up, but it did so just fine. Looks different, but penetrates like crazy now.