You've got several options. First, all these options except the homemade route are on the first page or so of the Sovereign accessories sticky and links to some of the Findmall sponsors that carry them are in there too. You can do it several ways...
Either add your own homemade shaft extension to the back of your shaft and then drill your stock mounting bracket rivets out . It is made up of two pieces and it's needless to use the half that doesn't clip the box to it as that's just for mounting between your stock arm cup. Then you can then take the clip portion and then re-rivet that to the extension you added at the back.
I took my clip apart on my homemade land rig and riveted the box clip portion to my custom lightweight land shaft even though I was still mounting the box in the stock location on my new shaft, as I was using a lighter Whites arm cup to save more weight and thus it wouldn't hold the box clip on the shaft between the arm cup like the stock arm cup does.
The second piece isn't needed and I left it off my custom light weight shaft build. The arm cup will work fine without the bracket parts being held between the arm cup, as on my water rig I use the stock arm cup with no bracket between it as I am chest mounting the GT when water hunting.
Second choice is to pick up one of the commercial back shaft extensions from one of the Findmall sponsors that you can see pictures of in the Sovereign accessories sticky, and then do the above mentioned things to move your box clip back and install it on that.
Third option is to buy one of the commercial back shaft extensions that has the Elite/GT box mounting clip already mounted on it. Pics of that type can also be found in the Sovereign accessories sticky from Findmall sponsors. I think there is also a pic of a back extension in that sticky for older Sovereigns that use the V-clip mounting system for the control box (what's used on the Minelab meters to this day).
Oh, one more option. If you want to make your own back shaft extension but don't want to drill the rivets and take apart your stock box clip mount, you can just buy another box clip mount on it's own for the Elite/GT and then modify that by drilling out the rivets on it to mount the clip portion to the back of your homemade back shaft extension. There are also pics of the stand alone box clip you can also get from Findmall sponsors on that accessories sticky too.
One more tip- Some people who mount the box back there on an extension like that will flip the extension or the clip perhaps backwards on it, so now you can then mount the control box backwards which makes for easier access to the controls and PP switch in some ways better than even when it's mounted the right direction in the stock location on the shaft. On the second page of the accessories sticky you can see a few pics of a GT with the box mounted backwards like that, but I think it wasn't on an back shaft extension, and to me it makes more sense to do when using an extension.
If anybody is looking for these shaft extensions with the V-clip (the mount system used for the Minelab meters) that the older Sovereigns used to also mount the control box, I think there are extensions with that V-clip already on them too, or you could drill out/rivet your existing V-clip on the extension or buy a separate V-clip to use to do that. Any Findmall sponsor who is a Minelab dealer should carry those V-clips. For one I bet Ralph at Sunray does (he's got a forum here on Findmall for Sun Ray) because he uses those V-clips to mount the Sun Ray Probes on Sovereigns and also because he's also a Minelab dealer. I think he often has various Sovereign parts that other dealers might not have maybe. Shoot him a PM in his forum and ask, or you can try Kellyco, Indian Nation Detectors, and I'm sure there are several other Findmall sponsors who are Minelab dealers too.
). My email is mtldtktr@gmail(DOT)com