OK well my measurements *could* be wrong here, but I got a bit over 15.5 inches for the diameter of the false bottom. On the metric side of the tape it was reading a hair over 39.5 cm, which comes out to 15.551 inches. It is an extremely snug fit in the kettle, though, so I am not sure how close it's going to be in another kettle. Also, the bottom of the Blichmann is tapered slightly, and you'd want to set up something similar for the dip tube. The one on the Blichmann has a stainless steel washer the slides over the dip tube to cover up the hole in the false bottom where the dip tube goes through, and the way the dip tube itself slides and locks into place in the kettle is something I haven't seen on other systems.
One last thing--not sure if you've seen a Blichmann false bottom up close or not, but they just have a single standoff in the center, which is the same height as the taper height for the bottom of the kettle.