I’m Juan, we met about thirteen years ago in Spain after you had some issues with the batteries in your D2X. I’m currently in the UK finishing a degree in Mechanical Engineering so I might help.
CamDo seem to make some good housings for up to 600m deep but it’s only for the GoPro 3 and 4. It might be easier and cheaper to get one of these and strip the gopro electronics out of the body to make them fit the housing. It’s fairly easy, but not really reversible.
Alternatively, I suggest you look for deep light housings (make sure they are not oil compensated) and 3D print something to hold the GoPro in place inside the housing. If you decide to go that route just send me the dimensions and I’ll do it for you FoC.
They are out of stock but this would have been great (and it goes as deep as 1250m)
Cam-Do is from Canada so you might want to give them a call as they might have some spare parts left.
Another option would be to use off the shelf ROV parts such as the aluminium parts from BlueRobotics:
They are only rated to 400m but there are other options so that would be a matter of hunting around.