The hot rod can counteract a backbow. That's why they are made of two threaded rods, because there is mid-point where there is no tension, but it will bow in either direction. The flat top dual rods just correct a forward bow, and the nut will go loose when there's no more tension. The hot rod is a good rod, and I used it for one wizard style neck, but I took about 1/32nd off the fretboard thickness to put the rod more in the center of the neck. The thickness at the bottom of the channel is of little consequence since the rod will not be pushing against that part of the neck. It will be pushing up on the fretboard and back on the heel and headstock area. With any non-traditional dual rod, I recommend having a volute. There is no reason not to. You need extra meat there, but not necessarily under the rod along the back of the neck.