This is one of my solutions as fitted to my Unimog 1450L DoKa. A fully rotating hatch that runs on a ball bearing race (continuous 360º traversing), the internal diameter of the hatch is 24 inches so big enough for most guys to fit in comfortably. Not shown in the photograph is the camera mount which bolts on opposite the hatch cover.
Construction is mostly aluminum but weight is still around 50kg and the rotation is super smooth. I have to say these are not cheap as there is a lot of engineering in them, they are waterproof and the hatch itself is countersprung so easy to raise and lower, plus it locks open in various positions. If anyone is interested I can sell you one (PM me)
It will easily support the heaviest camera and lens combo you can find (other than astro scope cameras) or afford. I use a Canon EOS 1Ds Mk2 with a 600mm f4 with no problem.
best regards from David Gudmunsen