What about just plain painting on fabric using fabric paints. You could thin down like watercolors and they'd go pretty far. Then you could just use a big flat white sheet. Pencil your grid onto the sheet, cut the sheet to the size you want, and then just get after it. A lot of your detail could come from your quilting in the water areas and trapunto could add height to your islands. You're way more creative than I am, but it might work.