Read your title and was thinking someone bought 3 Jubilees for a full frontal assault!
As of this moment, I don't think I've heard of anyone marrying a Cornwall bass bin with a K510. Though there is a passive for using the K510 with a LaScala bass bin, I'd suspect it would be different when used with the Cornwall...as such...I don't think it's been designed (by Roy anyway).
Using an active would allow you to tinker with it to get it right... or at least closer.
I'm here to tell you the K510 is a fine sounding horn. Go balls to the wall.... if you're going to build.... build either a Jubilee bass bin or on a smaller scale, a LaScala bass bin and slap the K402 on top!
You would have known crossover values here as well as existing passive designs.