Eddie,
The "One box" project sort of worked as long as it was one for lochs, one for rivers, one for salmon, one for tubes and one for salt!
The salmon doubles and singles box was the easiest to do. The tube fly box was not so easy but after a season it got pruned back brutally and I am now happy with it. It doesn't get used a lot and on the last couple of days on the Dee, Beauly and Ness it stayed in the bag and the doubles box was my only companion.
The trout loch box was easy but rather than carry a separate Rainbow box I amalgamated the two, something that might change for next year. Another wee box full of buzzers, lures and reservoir dries might have to happen. This would free up space in the loch box for a couple of "prototypes"
I do think that if you present two flies to Highland trout and one of them is black and the other is claret then you can't really go wrong (most of the time)
Right now I am very happy with where I am and feel that I could go anywhere in the UK and have something that would catch most species. Maybe not specialised, but able.
I have also sorted out a lot of my tackle for the tail end of this year and the start of next year to suit the simple box approach, but that will be a different thread.