I used 2x4s horizontaly, just to get me the extra 2". I have 2 center braces on the front, just for doors, and 1 center brace on the back. 6' of glass is a long span for no supports
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ps. Don't look at the canopy I decided to change the design after I built it so I didn't sand or stain. I don't have a shop so I build everything on my deck. (It's a little hard to do in 2' of snow)