We found exceptionally clear building instructions for both Mountain and Hammered Dulcimers from MusicMakers.com
The 17/16 Hammered which my husband built, turned out to be a very good sounding instrument. In the catalog, it is listed as "4 hammers" out of 5 for difficulty but if you have patience, the instructions are very clear, the pictures/drawings very clear and if needed, the team are very approachable and helpful.
This company has instruments which are complete, or in kit form, or full size plans with detailed instructions. We chose this option and the plans/instructions were $25 plus shipping. They also have plans for adjustable and sissor stands as well as plans and kits for harps, mandolins, bouzouki, mountain, hammered, finger drums, etc. Check it out, it may be what your looking for.