The 14.5" seems familiar, but a lot of photons have passed through the exit pupil since that one. Seems like I was helping a guy out that was considering a modified DK. I got that so-called Baader design from a post over on the Yahoo Astrograph group. Still not sure about it, but it does work beautifully on my 16" f/6 Newt. I think when you get into wide fields and lower focal ratios, though, no generalized commercial corrector is going to work well, and a custom design is required.
Telescope services do one Mike which they say will work down to F3 but it's rather expensive and long so would only be of use on an astrography with the camera on axis.
There is also a Bader to F3.5 as well as the other one they do
There is a syntra / skywatcher one to F4 around on there as well. Long again which means huge central obstruction I suspect.