Nice rig! Always like to see people's DIY ingenuity of what they want in detecting gear. Just polished off a few things on a waterproof Sovereign GT project built last year that needed some tweaking (video below).
Since the GT is my go to detector for old coins at "dead" sites on land I needed it to be quick and painless to remove the control box for easy transition back to a light weight custom land shaft (1.5 pounds lighter than stock). I use the stock shaft as my water rig.
Also wanted some convenience features, such as several ways to configure the Plano box depending on hunting circumstances (tether/float, hip, chest, shoulder, or back shaft extension), but prefer a snap on back shaft extension to eliminate float, drag, or risks of leaks by it being kept high and dry.
Have a Mode swith for things like pin point mounted inside both rig's grips that quickly plugs into the control box and retains full access to all modes rather than just flipping PP on and off. The stock mode switches still work whether it's plugged in or not.
Easy enough to find switches applicable to older Sovereigns or the Excalibur (simple on/off switch to toggle PP on and off) but that wasn't going to work for my needs. Couldn't find a waterproof switch with the proper configuration, so built one.
Next project will be getting another Sovereign control box and stripping out the guts to build a custom sleek waterproof enclosure with external pot and switch controls.
There's a lot of dead air space in a Sovereign control box so I'm estimating it'll be 1/4th the size height wise and roughly a few inches shorter.
Going to mount the external controls on top for easy access, but it will also still have a remote Mode switch inside the grip. That one is going to have another surprise haven't seen done with a waterproof Sovereign yet.
The video just now did going over a few of the features of this current rig can be found on Youtube here...
http://www.youtube.com/channel/UCbajcmikUXnYxkAThsCOwdQ/videos
The next video will show the recorded build steps and cover additional features.