I certainly wouldn't make any assumptions about a different kit working. My 2000W Honda uses a fuel pump, for instance, so I had a little more options when I made my external tank. It will pull fuel through the line, from the external tank.
But for gravity-fed carbs (which seems to be the Predator 3500W type), it gets a little more difficult. The tank needs to sit above the height of the generator, for instance.
This kit (about $200, shipped) says it's for the 3500W Predator, as a ready-to-go option:
You can also buy just a cap:
But you'd need to supply your own tank, fuel lines, etc. Small outboard-motor gas tanks are often used.
Have you contacted IPI Industries, the people who make the BERG systems? They could probably tell you if they make something compatible with the 3500W.
I got a used 6-gallon outboard tank for my setup. I modified an original Honda gas cap, to add a barbed connection for a 1/4" fuel line (found 1/4" rubber fuel line cheap at Lowes, in the plumbing section of all places). I put a quick-disconnect on the fuel line between the cap and the tank. And the tank's primer bulb lets me fill the tank->generator line with gas before starting. With 7 total gallons available, it can run my EU2000i for 28-70 hours, based on Honda's statement of 4-10 hours/gallon, based on electrical load. It was a bit of a project to rig up, but it's a nice capability, and will let it easily run overnight if needed, without worrying about running out of gas.