I agree with Dan's suggestions.  Sustain comes from a rigid structure and light vibrating components.  In my Aaron O'Rourke Fingerstyle dulcimers I use a fairly thick top (up to 0.150) to limit the sustain so the sound doesn't get muddy with fast fingerstyle play.  The sides and back are 0.080 thick so they do add the the texture of the sound.  On soundboards, I measure the longitudinal and transverse stiffness of the plate and sand until I get the stiffness that I am looking for.  The stiffness is a lot more important than the thickness.  There is a lot of useful information in the online book "Left Brain Lutherie" if you are inclined to venture into the technical side of instrument making.

I don't measure! I "feel of" the wood panel as I'm thickness sanding to the right thickness. Flexing the panel and from experience knowing the right mass I'm looking for for a particular tone. Harder denser woods can be sanded much thinner than a softer less dense piece.

As for sustain? Panel thickness is just one variable factor in a multitude of factors involved. String selection, nut and bridge hardness or softness, nut and bridge placement, over all mass of the piece, wood selection, possibly the finish, intonation, and so on.

If sustain is what you look for, I'd try harder more dense woods sanded to around .090 if you can, harder nut and bridge, and possibly a zero fret?

Some place to start....

Debbie, thank you.  I do not have a website.  Building dulcimers is a primarily a hobby for me.  What you see is the second of my builds. About half of my builds are donated to local charities such as Habitat for Humanity and Pink Gala. The rest of my builds are commissioned pieces simply generated by word of mouth.  Being this is a hobby, and I prefer not to work to deadlines, I don't promise a finish date but usually have them completed within a month or two. It is nice to be able to enjoy retirement at my own pace. Other posts of mine contain photos of some of my other builds; each one is a unique/custom build. 

Most of my work starts with 3/4 rough sawn lumber and I work it down from there with a combination of using my surfacer, resawing, and horizontal drum sander, and then hand sanders. Thanks Ken, I'll try to find those ED discussions.

Robert, I still use three thin cross braces but am going to try going to braces that don't go across the entire bottom.  Usually my bracing is only for support of the bottom and doesn't touch the top as I rely on the fret board for that support. I do hollow the fretboard to provide more 'open space' inside the instrument; minor addition but I believe every cubic inch helps. 

My usual top and bottom thickness is about 1/8", but I try to go thinner than that if I can.  As you might have read on this site, or elsewhere, you can get get good sound out of a Tennessee music box even when the top and bottom panels are over 3/8" thick.

If you can find the old ED Discussions by Richard Troughear called An Interesting Dulcimer Experiment, he may have researched and reported on the phenomena.  I use 1/8" normally because I can readily get wood already thicknessed to that dimension.  Sanding doesn't take off much from that.

I use 1/8" . I tried. 3/16 on my first dulcimers but didn't sound as good. I would love to try 1/16 but then you get into bracing, which might restrain sustain. I think thinner wood would be better if only it were stable... Robert.

I did a search but nada....    so, the question is:  What effect does the thickness of the top and bottom of the instrument have on the length of sound sustain?  Secondly, what thicknesses do most of you prefer for your tops and bottoms?  Right now I am working at 1/8" to 1/10" but mostly 1/8".