I have found that Lows and Home Depot carry soft plastic spacers for laying tile. They come in a varity of sizes and are shaped like a Plus sign. I take a razor knife and cut one leg off to make it T shaped. After applying my goop i use 5 of the T shaped spacers evenly spaced around the glass to keep the sound board centered and a spring clamp to keep it held tight and in place until it cures. Just thought I would share my .02 whille we were talking about it...
Dennis