You can't use standard canvas for the reason you pointed out - you really want a porous material, otherwise the sound just bounces off the canvas.
Have a look at the Spoonflower website - they can print images on fabrics. Fabrics are porous and allow the sound to pass through them - some better than others.
If you Google Spoonflower, someone even spent the time to review the various fabrics on offer at Spoonflower and which of the bunch makes the best sound panel! How cool is that.