I'd try a local fabric store like Hancock's or Joann's - they have several different weights (thicknesses) of clear or translucent, maybe even colored vinyl that you can sew in - hmm, maybe even stick some velcro on the wrong side of the vinyl liner and the inside of the bag, so you can take it out and clean it if need be. They should also have iron-on velcro - that'd make it super-easy!
Do please show pics when you're done! Sounds great
