Have you tried Mosaic Mesh?
A plastic mesh of approximately 2mm which holds mosaics rigidly in position when tiling. Used by both Professional & DIY to make mosaic fixing fast and simple, and to give a fully satisfactory finish.
• Self adhesive, easy to apply, no hassle • Keeps mosaics evenly spaced, no need for a multitude of spacers
• Easy to cut with a knife, no loss of mosaic versatility
• 300mm x 300mm, fits a standard mosaic sheet
• Eliminates mosaic alignment problems making fixing even faster