Did you know you can put a mosaic intended for outdoor use onto styrofoam? A friend of mine mosaciac'd a large rectangular piece of styrofoam she had cut at Home Depot. She mounted it on backerboard with Vulkem and hung it out for several months to watch what might happened. Well it held up nicely and I think is still out there. The styrofoam gives it a nice "frame"/depth without the added weight if you used something else as thick. I just have to try this. We need something out on our back fence, this may be just the touch. Just thought I'd pass along the idea!