Have you built any other 12-string basses?
I just started putting together a parts list for one. It will have a Warmoth 4 string 13 degree tiltback paddle headstock neck with carbon fiber rods. Warmoth's 4 string necks have the same heel as Fender 4s but with 21 frets. The Big Bottom 12 is stable after almost a year under tension. I haven't had to touch the trussrod.
Body will probably be a version of G8R, MAX, or ExplorerBird.
I sent the G8R to its new owner Friday, so I'm ready for a personal build.