The other option is to silicone on 2 thinner plieces of glass instead of one 1/2" piece. 7mm strips would be cheap. I have a bunch of 1/4" plate glass from windows you can have to cut up for free if you want. 
 
There's a local glass shop near my place that would probably cut what you need for $20 max., probably even $10, out of my 1/4". If you let me know how big a piece you need for both breaks, I can see about getting them to cut my glass for you. 
 
Anthony 
		
	
		
		
		
		
			
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