Just found this thread and pleased to see that for once I'm ahead of the pack. I just fitted and used on a trip to the Northern Red Sea, a Housing Sentry from Bill Libecap. This is a two part electronic version, Bill kindly tailored a valve assembly for me to fit in the threaded housing hole of my Ikelite D300s housing. Now I know that Ikelite are not the most ergonomic housings, I try to set the camera up as far as possible and not alter settings underwater, and so I seldom if ever use the control for DOF and the F button on the D300s also the push rod used for those controls is the single thing making camera installation difficult so it was obvious to me that sacrificing this control was a minimum risk installation strategy. It took about an hour to install worked first time and has given a huge increase in confidence in use after a serious flood 3months ago. This flood was not I think user error but occurred in a fast current on the surface while descending a shot line, I believe the long macro port attached got knocked and the current flow forced water past the oring that on the surface was not pressure sealed. The vacuum device should and has on 18 dives so far prevented a flood this has included backward roll RIB entries and occasional giant stride of the back deck.
Re releasing vacuum and opening housing causing a few drops of water to enter, yes I have noted this and it is important to rinse and dry the outside prior to opening but not a major problem.
I would echo the sentiments about Bill being easy to work with and get what you want to install in your camera, thanks Bill