Yesterday I was at the gas station and I spotted a guy with what looked like a cigar box guitar headstock coming up out of his backpack. I walked up to him and started a conversation, and sure enough it was a homemade guitar (not a cigar box, but he used a box that was about the same size.)
[Just the headstock on this thing was so cool. He used eye screws for tuners; it was pretty badass.]
That got me thinking about the cigar box guitar I made several years ago. Sadly it perished in the basement flood, but I did get a few videos of some friends playing it up on my Youtube channel. Here's one of my friend Ian giving a demonstration on finding the full scale on the CBG... but it applies to any fretless instrument, really.
This CBG was tuned to open D tuning, if I recall correctly. D-A-D, I think, with the low E dropping down to a D, then A, then D. Simple and easy to set up with any regular set of guitar strings.