Hi @kurtnelle, welcome to the forum 
Blue Robotics only sells up to 8" diameter enclosures, and 4" diameter domes. You’re welcome to use our drawings and CAD models as inspiration for a larger design, but you’ll likely need to make your own design and source the parts yourself.
It may be helpful to refer to the Under Pressure pressure vessel design software for determining suitable tube and dome thicknesses for the depth and water type you’re interested in filming in.
I would note that the Black Magic Pocket Cinema 4K is 178.1 x 96mm, and has rounded corners, so it should fit into an 8" enclosure if it’s ok to use it without the cage.
From a quick search online it seems that camera cages are generally used to make cameras easier to hold and to protect them from falls/damage. If it’s in an enclosure then you don’t care about holding it, so it’s perhaps acceptable then to just get some a mount/some corner pieces made (3D printed?) to support the camera and protect it if the ROV/enclosure somehow gets dropped.
Using an 8" enclosure would still require purchasing a custom dome, but at least that way you could purchase pre-made flanges, o-rings, tube, and the rear side end cap (depending on your depth requirements).
Hope that’s helpful!