This is why I like this forum so much, lots of good info and leads.You could convert a resonator tenor guitar by Gold Tone or Ozark, but neither is particularly small body-wise, and nut width is very narrow at ~32mm (I guess it could work converted to a 3-string, EAD-tuned bass):
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MBT+: Metal Body Tenor Guitar (Discontinued) | Gold Tone Folk Instruments
These resophonic octave mandolins would be sweet, but not cheap, as modding platforms go:
Gallery: Mandonator™ 8 - Resonator Octave Mandolin-Mandola - Topher Gayle, Luthier
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