One possibility is to buy a pre-slotted guitar fretboard and cut it down.
You find a 12-fret spacing which is half the scale you desire - i.e. if you want a 19 inch scale baritone, you find the 13 frets on your guitar fretboard which most closely span 9.5 inches (mentally number them 0-12). This is probably frets 3-15, 4-16 or 15-17.
Your nut or zero fret is at your 0 fret, so cut the board off to accommodate that. It's probably too long at the other end as well, but save cutting that until you've built body and neck so you can decide what length over the body looks best.
Finally, mark its new width and plane the sides down to those lines. Voila! baritone fretboard.
This works because the ratio of each fret spacing to the next is constant. So whichever fret you cut at, the 12 frets following are half your new scale length.