If it's something you're gonna be worrying over, just make a brace/bracket out of steel or wood. You really only need a piece of material with channels cut out where you want the glass to slide into. The brace will keep it from spreading. I'd go with steel/aluminum over wood for this application. You can even buy the steel pieces at Home Cheapo, and just channel it out with a grinder/metal rasp/ hack saw to allow the glass to fit into. It won't be the most attractive but you can paint it too.
Just a thought.