Obviously, this forum is, if not dead, moribund.
Building a switch rod is no different from any other fly rod, except for one thing. You need to add a longish fighting butt. I would suggest one that is 3" to 4" long, depending on the size of your hand. This becomes the rear grip, when you do a spey cast, so it needs to be comfortable, but not so long that it interferes with single hand casting.