You could go to a craft store and buy a spool of wide (maybe like 3 or 4 inches)white ribbon and paint/stencil/iron/whatever the words onto it. All you'd need is one strip long enough to go from your hip up to your shoulder and back down to your hip where you can just pin it closed with a safety pin.
I'd use ribbon instead of fabric that way you don't have to worry about finishing the edges.
Oh! Sometimes florists have really wide ribbon that they use in large floral arrangements. You could try asking local florists. I once had a big reeth made and they wrote 'congratulations!' in glitter glue on the ribbon.
You could even try a party store they sometimes have sashes. They'll probably say things like 'bachelorette' or 'kiss me I'm 30!' but you could just turn them err...inside out/over and write on the blank side.