The RAI does not improve wading
So after setting out on my research to find out how to make the RAI watertight, it appears i didn't end up with the answer i was looking for.
Effectively the RAI is not meant for wading at all, and so even trying to waterproof it seems to have little value.
Please see this thread where I have mentioned the feedback i received from LR and Mantec, who make the RAI. (this might also suggest that a snorkel would also not improve wading).
http://www.disco3.co.uk/forum/topic118283.html
So as I was going to run my cabling up the RAI, or if not at least use it to hide the cables as they come out of the A pillar trim, this now seems to be about the only value that it adds. Seems expensive at about $900 delivered with the other two parts required for the D4 3ltr, when it doesn't really add much functionality.
Now i am debating whether i get it or not?