I buy sewerage/waste water endcaps. They about 100mm and on my 4300c cameras which are 100mm diameter they're perfect and pretty cheap. Then as you say drill for the
cat5 then crimp the end and bundle the tail and cat5 into the end cap.
I just bought a 4421 and this is 110mm so I couldn't use the same end cap, managed to find a 150mm one. But it's about the same cost as an official dahua roof mount, I only went with it as I wanted to install that weekend and not wait for shipping. If I bought another 4421 I'd get the mount at the same time.