its a beautiful quilt - great job! I personally am thinking something batik.. but not too strong so it doesn't over power the top.. but then again sometimes a bold backing is awesome.. see I am no help
not sure if batiks come in wide.
Not sure if these links will work - but these I love
https://www.fabric.com/buy/0459270/w...s-flourish-tan
https://www.fabric.com/buy/0433002/w...-skylight-blue
https://www.fabric.com/buy/0471409/k...-stripes-water
https://www.fabric.com/buy/0471410/k...-stripes-retro