I've used 6-8" tamborines and removed the jangle disks from the sides.
Remo. I just prefer to play the banjo rather than try and make a dulcimer sound like one. Banjo's are mostly tuned to an open chord and can be played with one and two fingered chords, so why try and make a dulcimer sound like one. I like the sound of the dulcimer as is and the banjo as a separate sound. Get a banjo and solve the problem all in one whack.