Here is a simple arrangement of the blues classic "Statesboro Blues". I've got the 4 string fretless cigar box guitar tuned to open G (G-B-d-g) which makes it easy to play the song on a single instrument and still manage to add some Duane Allman style slide.
While the song is played in the key of D, Duane Allman had his guitar tuned to open E and played the song around the tenth fret. To match what he did I like using the open G tuning G-B-d-g and centering my playing around where the 7th fret would be (if I had frets).
While this open G tuning may not be as popular as other open G tunings (namely the D-G-b-d open G tuning), I think you'll find it great for call and answer blues riffs