I use my Cole jaws. If the rim of the bowl is too warped to sit flat then I use my electric plane or homemade disc sander to remove the high spots so it'll sit flat. I often will only use four of the blocks (vs all 8) to hold the bowl if it's too oblong. Since I'm just trying to make the tenon or mortise round it works without problems for me. In the future, now that I've remade my disc sander I'll use that to flatten the rim every time.