The web is a wonderful thing! I stumbled upon this site the other night and thought I'd mention it here. Authored by a rather amusing chap named Thomas Price, (a self confessed "Galoot") it details the design and making of a 15/14 hammer dulcimer that he designed one Christmas to avoid meaningful interaction with his in-laws.
Despite the disclaimer that this site does not have all the information for a total novice to build their own instrument, it does comes close, and offers a good deal of useful information. Thomas really doesn't give himself enough credit; I suspect anyone with basic woodworking skills and access to the tools needed could making a functioning instrument from these instructions.
He also kindly gives measurements in both the English and Metric systems, nicely done diagrams and has a list of useful (and current) resources and supply houses for parts, pins strings, etc. and does it in a humourous, approachable fashion.