Stephen concluded (although I'm hesitant to speak for him) that the best extension is a 57.5mm, and you can buy an extension ring that size.
I decided I didn't want to carry an additional extension (especially one that long) around, so I've settled on stacking a 30 and 25 to get an effective total of 55mm extension (haven't actually shot with it yet, but have the rings). According to Liz, Stephen thinks that's a better option than 60mm, but he still considers 57.5 the "best" length. He can correct/clarify if I've misstated that.
Also, my impression is that for sharp images with that lens, using a fairly high aperture (f/8 or above) is as important (if not more so) than using the exact "ideal" extension length. IOW, I would expect better results shooting at f/11 with my 55mm extension than shooting at f/2.8 with a 57.5mm extension. Of course, the ideal is to use the "best" extension AND camera settings, but if you're going to fudge with one (as I am), at least put special effort into not fudging the other...
Hey Guys - Sorry for the delay, but I'm in PNG at the moment.
I did decide the 57.5mm port extension is the best (with superdome), and Seacam now offers that as a standard item from any Seacam dealer. Actually, it was splitting the difference between the 55 and 60, and like Bruce said, if you already have a 55, or a couple that add up to that, that should be close enough. The PVL 60 gives so much upward torque from the superdome and that much extension, it is hard to handhold a strobe and use the housing in one hand. Therefore, 55 becomes the far better choice. However, starting from scratch, and not being opposed to carrying a port extension that long, go for the 57.5.
As far as diopter, I gave up on that test because I could never find an 82mm slim enough to not vignette.
BTW ... a working aperture of F-8 and smaller is better, for sure, and if it means working at ISO 200 instead of 100, it is worth it for that lens.
BTW2 - I had a big edit recently that forced me to look at a lot of older images, including 17-40, 16-35 (original), Nikon 12-24, and the new Canon 16-35II. I like the 16-35II better than the others, but the corners will still be a challenge. A manual focus gear is a nice addition for some subjects, so you can more precisely cheat the point of focus and try to cover more of the corners + a central subject.