A change of string gauges will give you options for different open keys. There is a string gauge calculator for this. http://www.strothers.com/string_choice.htm
Thanks, Jim. What I was trying to get across, and badly, that what I need are plans for such a critter to do it properly. Due to my infirmities (age, crotchityness, and very large hands that have some arthritis and don't bend like they used to) I need to construct something with a wider neck on a body that will sit on my lap or desk, and will sound decent. the bends and curves are not necessarily required, and a box will be more than acceptable. Don't ask much, do I. Thanks for the response. Lee
When you get your head turned around, You can decide what string spacing feels better with your fat fingers. That is a fairly simple adjustment. As far as dulcimers set up to play in any key, good luck.
Most dulcimers can be tuned to several different keys. For instance a dulcimer tuned for "D" can be tuned down one or two keys until the strings get to "floppy" to use. This means you can tune a DAD instrument to CGC or even BFB, but pretty soon that bass string gets too loose to give a nice tone, or feel when you strum it. You can also tune the instrument up one or two keys, You can usually determine the limit when your melody string keeps popping (breaking). In most cases you can tune upward to EBE and maybe GDG.
Now, if you are building your own instrument, you can adjust the scale length (VSL) of the instrument. If you want it more bassy, increase the string length, or more trebly, decrease the string length. If you cut off say 4 inches from your string length, you'll find you are in a new "ball park" and can tune it way up. I assume adding another 4 inches to the string length would allow you to reach some much lower keys. Just think of how a bass guitar sounds verses a standard guitar, verses a baritone ukulele, verses a soprano ukulele. Much of the difference is the scale length, but some is due to the strings. In fact, changing string sizes can also allow you to reach lower or higher keys on your current instrument as already has been mentioned in this thread. I have an article on building lap dulcimers which you can get to by clicking here. The article should be of interest if you are thinking of constructing a dulcimer.