[QUOTE=Chas Freeborn] Construction differences are relative to the additional string tension, etc.
-C[/QUOTE]
Tim,
Even here you don't have to do anything radically different - you can design around scale length and string gauges that give you the same tension on more normal scale length guitars. In fact this is one of the beauties of these guitars. You can tune down differently with different string gauges. For example on the 28.9" scale baritones I make you can string with 0.012"-0.053" and tune down one step (D-D), or 0.013"-0.056" and tune down one and a half or two teps (C-C or Csharp-Csharp) or use 0.014"-0.070" to go down four steps (B-B). All of these give very different tonal ranges but have the same string tension.
A-A is probably where most baritones aim for, but in my opinion you need 30"+ scale length to make this work properly. The lowest string is always the problem as you are into the same issues/problems as acoustic bass guitars unplugged.
The one thing you will have to alter is string compensation on the saddle for the thicker strings. I use a very heavy rake back with a split saddle (see pic). You will have to experiment here, it all depends on scale length and strings used.
Wood choice is also important as you need to keep string clarity with the lower tones and avoid muddiness. David Bekowitz advocates mahogany, koa and walnut types of wood for B/S and I like Euro spruce/Cuban Mahogany. You can make it work with rosewoods but have to pick the right set.
The other difference - that sort of comes with the scale length - is body shape and size and where the neck joins the body. A lot of makers base their baritones on dreadnaoghts, but I prefer a more curvy OM/Small Jumboish shape. My baritone was based on my 12 fret body join, 25.75" scale length guitar. With the 28.9" scale length the same body became a 14 fret join baritone. I did some research on this and found some interesting relationships between the scale length of the guitar when capoed at the second fret - which if the guitar is a 14 fret body join turns it to a 12 fret body join equivalent:
14 fret scale 12 fret scale (i.e capo II)
29" 25.75"
28.3" 25.3"
28" 24.9"
27.5" 24.5"
26.5" 23.5"
These are some interesting and familiar 12 fret scale lengths. If you combine this with the set of strings used for the one step tune down, then playing with these and the guitar Capoed at the second fret you are playing different 12 fret guitars with light gauge strings in E-E tuning. In the case of a 26.5" scale it would be like playing a 12 fret parlour guitar.
The body design principles are the same as designing 12 and 14 fret guitars with shorter scale lengths (or 13, 15 and 16 fret body joins for that matter!)
I have some stuff here on my website if it helps.
Baritones