My next project will be something of the sort. Not sure what I'm going to call it yet... maybe octave harp ukulele? 8 string instrument (4 neck, 4 harp), laid out like my harp ukuleles but tuned an octave lower (CDEF GCGC), or up to G or down to A using different string gauges. Not a super versatile instrument, but super fun and easy to play
String lengths range 29" to 20". I drew up one 14" lower bout version and one 15". I like the 14" better. Soundboard length is 29", but you could make it work with 28.5" by lengthening the harp headplate.
I haven't made a printable vector plan yet, but I could if you want to build one.