Hello dert,
Some here on the forum can modify a GT or a Sovereign into a waterproof detector, Some do it for pleasure and enjoy building things so I see the justification with those that modify or build something from the ground up. But honestly it's too much trouble and labor intense to construct one reasonably for someone else and if you factor in the labor of building one from the ground up it'll far exceed the price of a new Excalibur.
Plus, To construct one from the ground up is time consuming. To locate the correct waterproof housing, waterproof pots, waterproof cable connectors both coil and headset, Waterproof headset and then strip down the circuit board removing all pots and switches and rebuilding a new wiring harness takes time and can be very expensive, This is the correct way to make a waterproof detector so all controls still function. Might as well buy the Excalibur and save you headache and $$$
Mounting the housing inside a plastic waterproof clam shell container with only the headset and waterproof coil connectors will work but it's a nuisance having to open up the clam shell to turn on or off or even adjust the controls.
The Excalibur will cost you about the same maybe a little more and you can use the Excalibur for inland detecting. Many users use the GT or Sovereign inland without a meter, It's basically the same as using an Excalibur for inland.
I don't mean to sound negative and I've built a few waterproof detectors myself, Maybe someone out there can build one for you or already have one they would consider selling.
HH, Paul (Ca)