Here's what worked for me. I bought the lower Weber charcoal grill which was about 10 bucks at the Depot to be the upper for my large. Then I bought a cast iron 18.5 inch on Amazon from Music City Metals http://www.amazon.com/gp/product/B000HKI2YY/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1 for the bottom of my large. Then I took my original BGE grill from the large and used it for the top on my XL. Doing the math and including my Medium I now have 9.4 sq ft of BGE grill area!
XLBGE, LBGE, MBGE, MINI, 2 Kubs, Fire Magic Gasser