Stephen, how about SinghRay for a custom diopter. There main business seems to be neutral density grads, but they may have the wherewithal to do a diopter, and at least they are relatively local to you.
Thanks for the thought Craig, but I have the Heliopan on order. It should show up within a couple of weeks so I can complete my testing. Meanwhile the 16-35II is certainly usable without a diopter. Very sharp in the 20-35mm zoom range and actually probably not a lot different in the corners than the 17-40 Canon, or Nikkor 12-24 for that matter. But definitely not perfect. Here's a shot I took last weekend. The lens does reasonably well on subjects on the same plane. (Seacam superdome, PVL35, F-6.3)
The challenge is with a center focus point somewhat in the distance and dimensional subjects in the foreground that reveal the soft corners. That's the aberration I hope to be able to correct.
Also, I've ordered a manual focus gear for this lens. I think there is a sweet spot between a subject in the distance one might choose for AF, and a focal point that might hold both foreground and primary subject. Rather than constantly shifting zones of AF, I think I'll go back to CF 4-1 on Canon and MF as needed.