Great job and very cool setup! I hear you can get at least certain Plano water proof boxes at Dicks sporting goods stores, and I bet at marine supply/boat shop stores as well.
Some are using a longer Plano box, while I believe the one you are using people are mounting the clamshells at the bottom of the box maybe? I think the one you've got there is the size I've been eyeing others using that I plan to use. More compact. Doesn't really matter, as my Plano box is going to mount on a snap on back shaft extension on the stock GT straight shaft, which I only use in the water these days since I built a light weight land shaft for land hunting with the GT.
I used to chest mount the GT in the Minelab bag for water hunting but was limiting to only feeling safe to about my belly button, and even then had to constantly remember to not bend over when scooping. A few close calls. I hate chest or hip mounting anyway, so that's why I want it on a snap on back shaft extension. That and also so it's now at head height that way and no risks of leaks with the box, as even neck deep the box will be a bit out of the water. Be careful with those boxes. I hear they leak around the lid a lot. Another reason I don't want the Plano box in the water, and also because of drag or float issues if I had it on my body or lower on the shaft.
There are little moisture alarms you can buy at home improvement stores that run on batteries and are meant for sticking under hot water heaters or the kitchen sink to sound an alarm if they sense moisture. I won't be needing that for my setup but for sure if the box is in the water then I'd would recommend using one. Also, adding some thin foam material like used for yoga mats might be a good idea to both cushion the GT from bouncing around inside the box and also to suck up any water before it can roll around in there and find it's way into the box. I plan to do that with mine on the bottom/sides, but not the top because I want to be able to look down into the box to make sure it's not wet should I take a fall or get hit by a rouge wave or something.
I'm not using clamshells. $20 a piece is kind'a pricey, although I hear you can route both the headphone and the coil cables through one clamshell if you drill the holes right. Both need to be far enough apart so the black foam seals around them, yet not so far apart that the clamshell plastic outer shell is in the way is what I hear. Instead, I'm going to seal the coil cable for the 10" Tornado's hole in the box with Shoe Goo, and also seal the headphones that way as well for it's hole, as I don't ever use the 10" Tornado on land these days and also can commit a pair of headphones to the Plano box as well. down the road should I neither to remove either for use on land, simple enough to peal off the flexible Shoe Goo from the cables.
Headphones I'm not worried about being waterproof. If waves are cracking me in the head because it's so rough out I ain't going neck deep anyway. Just get beat around too much. Seen Kbron2000's videos where he's using Avio (I think they are called that?) wireless $42 headphones. Beauty of that is no headphone cable to waterproof into the box. He says they work well with the Sovereign with no delay in audio and such. Have to be careful with some wireless headphones. Many have delays in their audio compared to what the coil is doing, or they for some reason don't get along well with certain detectors.
Nice cat, and man is that WOT a monster in size sitting next to the control box. I saw one painted black with spray on bed liner and if I ever get a good used price on one and like it, I plan to paint it black that way too. Never cared for the color (wonderful orange thing), but the spray on bed liner pic I saw of one made it look pretty mean and also the textured finish of bed liner spray makes it look like it came from the factory black. Very durable stuff too so great for protecting a coil in the sand/water and also to drop weight and hassle of cleaning under coil covers.