Installing a Ring floodlight camera WITH junction box

I'm looking to fit a Ring Floodlight Camera - hardwired - either the pro or plus model in my back garden. We have an existing cable already wired next to our back door if we want to fit a light. I was going to run the existing cable through some conduit externally so it will fit above the door. The issue I have is the ring device itself doesn't allow for the hardwire to enter from the side, it has to enter from the back which then sits flush to the way. On the ring website it says to use a 4-inch round waterproof junction box. On the ring website in the videos demonstrating installation, it shows a junction box but they do not sell them. I’ve spoken to ring customer support who are not able to recommend a junction box. I've looked online at universal junction boxes and a lot of the reviews of people who have used them with ring devices all share they had to force the fit. Just wondered if anyone had fitted any and could recommend a junction box which was a clean fit with the ring devices, specifically the ring floodlight camera unit?