Hi Rob,
I've built a fair number of mountain dulcimers over the years and still build a few. Dulcimers come in many different sizes and shapes, but can be divided into two separate "camps" of construction - "zither" type and "guitar" type. The more traditional form is the zither type with the saddle located over the end block. These tend to be quieter because much of the energy is transferred to the back plate as happens with a zither, but on the dulcimer the back is damped by the players legs. The "guitar" type moves the saddle off the end block and on to the top plate, which allows it to become the main soundboard. Since it is not damped by the players legs these dulcimers tend to be louder.
Both types have the "fretbox" mounted directly on the top plate. In the interest of keeping weight off the top plate the fretbox is often hollowed out which also helps the volume of the instrument and when glued to the top plate this forms a box beam construction which forms a rigid structure to counteract the string forces wanting to "fold up" the instrument.
Dulcimers come in many shapes and sizes. A popular tuning is Dad, and with the availability of guitar strings scale lengths typically used by guitars are common. A guitar fret calculator can be used, with the accidentals removed to produce a diatonic fretboard (often the 6+ step (6 1/2 fret) is retained to provide an accidental in modern instruments).
A wide variety of woods are used in their construction and I have even used HPL (formica) for backs and sides. The typical scantlings are similar to those used for guitars. The size and depth of the soundbox is quite variable, over the years the tendency has been to make them wider (19-22 cm) and deeper (6- 8 cm) but all sizes and shapes are still being built as googling images will show.
If you have any specific questions feel free to ask.
A traditional and not so traditional sound sample:
https://www.youtube.com/watch?v=INu3UQ35yVkhttps://www.youtube.com/watch?v=yqHPaZZlEcs