Well after 2 years finally got around to modifying this dude this morning. Picked up an 80 CF lens recently, so now the modified hood can serve double duty, although not ideal for the 80. I stuck primarily to the overall length of the 38-60 hood, might be a few thousandths longer. After drafting up a crude sort of ray trace (I'm no optics expert) it looked like even the 38-60 might tend to vignette given a horizontal angle of view of 58.5 degrees for the furthest ray at the edge of the front element, so figured I'd just stick with this length to be safe and avoid trial and error.