THere's proabably a Weber made griddle insert specifically designed for Summit's size that will slip right in there without the need for fabrication. Weber calls their griddles "planchas" which I think is what they are called in Europe.
There are! I've looked into a few, but I decided to go the custom stainless steel route for a few reasons. First, I haven't seen any inserts that are so large that they would cover most of the firebox area, therefore I would have to use two inserts. Two inserts means there would be a space in the middle where they rest side-by-side. That small gap right in the center would be an annoyance, and an area where I couldn't cook. Second, the Weber inserts I'm aware of are made of cast iron. I love cast iron: I cook with a cast iron pan and dutch oven quite frequently. That said, it can rust and it is heavy. This grill will be kept under a cover, but outdoors. I'm concerned that a cast iron cooking surface would be prone to rusting if kept outdoors.
Cost-wise, my buddy is selling me the stainless for the cost of the materials, nothing more. He estimated less than $100. I don't think there's a major difference in cost between these options.
Cast iron's advantage is that it conducts heat really well. However I'm thinking that advantage is unnecessary on this grill, as it has four burners (they run front to back). I believe these will provide plenty of heat across the entire surface, so that I won't miss cast iron. Also, like cast iron, stainless can be seasoned in order to form a non-stick surface.
I'll experiment with ditching the flavorizers, since there won't be any food/grease drippings for them to flavorize. My theory is that the flavorizers intercept the heat from the burners, reducing overall heat output. On the other hand, perhaps the flavorizers help evenly distribute the heat. In which case, I would prefer an evenly heated cooking surface to one that has major hot spots. I'll have to test this.
Of course, my assumptions could all be totally wrong. In which case, hopefully this thread will become a cautionary tale for someone down the road considering just this idea.