I like the idea of making a fractured background of wild psychedelic colors with a black guitar silhouette on top.
To make the fractured background, you could iron strips of freezer paper together -- just overlapping a piece about an inch and ironing it into place to stay; you could glue or tape also. Once you have a huge sheet of freezer paper the size of the quilt, you could draw fractures (lines) at odd angles throughout. You would want to start with a few lines going out to the edges, to break the quilt down into smaller sections to be pieced. Number the pieces and draw registration marks connecting each piece so you can match them up later. Cut along the fracture lines (just one section at a time). Iron each paper piece to a piece of psychedelic fabric, match the registration lines, and sew pieces together.
I am currently reading "Fabric Journey" by Ruth McDowell so am heavily influenced by her method right now! I've never done this myself, but it sounds fairly easy. I think this would make a very interesting background for the guitar, especially if you have wild colors and patterns (maybe some tie-dye?).
If you have time, once the background is pieced and appliqued, you could cut off the edges and insert a black border before adding the cut-off pieces back. This would create an interior border to frame the guitar.
For this piece, I would *definitely* spray baste to save time and use a cotton batting or 80/20 batting (because spray basting holds so much better with cotton batt, plus cotton batting stands up to a lot of use and abuse over time).
Just my thoughts.....