On my last 12, I reasoned that, since it was pulling about twice the tension, the top and bracing would need to be about twice as stiff. Since the stiffness of the top is proportional to the cube of it's thickness, I made the top 25% thicker (1.25^3=1.95). The same holds for the brace height, so I made the braces 25% taller, and a little wider to get some added gluing surface. Other than that, and a wider bridge plate, the top was the same as I always make. The neat thing is that, with twice the energy going in, and only 25% more weight, you get lots more power. That was the first guitar I've made that I could not sing over.
I use a wide saddle on 12s; about 3/16". When I first string it up I have the top of the saddle flat, and, of course, the strings buzz like mad. When I've got the action height and relief set up the way I want, I slip a short section of B string under one of the strings, move it around to find the proper break point, and mark it. When you've done them all you end up with two almost parallel lines at an angle across the saddle for the lower strings: the front one for the octaves and the rear one for the normal strings. Note that the octave G break point will usually be forward of the high E break point, so don't start with the saddle too far back.
Once I've esatablished the lines, I file a V-groove in the top of the bone between the two lines, leaving the lines alone. The front edge is filed back to the front line, and the rear edge is rounded up to the back line. Then the ridges can be notched out with a small round file where you don't want the string to hit: notch the front ridge so the normal string can vibrate freely stopped by the rear ridge, and file the rear ridge to allow the octave string to ride off the front. If you do this correctly you can have all of the strings play with good intonation all the way up, which goes a long way toward improving the sound of a 12. It looks like you meant it, too.
I did use a shorter scale length on that guitar, mostly to preserve the octave G strings at normal pitch. As usual, I also made it with a 12 fret neck, rather than 14. I _need_ to make another one of those.
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) may do it along with lite guage strings tuned to concert. I hate harpy. My last 12 used .068-.014", tuned to a C+ and with a 25.34" scale. Top was just under twice as thick as I make a 6 string, bracing 3/8" and a HUGE bridge plate. Tone is FAT and fabulous. Leadbelly would have loved it. Next one on the table has a top just a bit heavier than that, a 26.22" scale and aiming for .070-.014" tuned to B.
