Glad you asked . I'm going to set it up as a standalone gridlle with a 1/4 grilling area similar to what @Jon Tofte did below. Once I get the weber crafted insert and griddle I'll cut down some 9mm SS grates form the Summit 450 to fit. Also going to add some lighted knobs because why not. Will also possibly modify the removable drip pan to a larger common and available size. Might look to replace the wheel axle as it's weak and flexible but that's only while the tank is mounted. I already purchased the tank cover or sweater as I call it as well as the Weber grill cover. I also cleaned it up a bit since it was used likely once or twice before I bought it hence the deal I got. Came out near new with some Dawn, soak, scrub.
So slight change of plans. I purchased the Porcelain Enamel Cast Iron griddle #7658 from Weber. Looks good next to the original Spirit II grates so I'll leave that way for now since they are new. Saving the old Summit 9mm SS grates for future projects. I also did some research and despite wanting to install the lights and knobs I have decided not too. This comes after realizing the Spirit II manifold is unique compared to the other full size grills that actually have them. The size difference of the stem is to small and even with some customization I didn't see it being worth the time. There was also the issue of have to find or make a mount for the battery pack. So for now I've accomplished the main goal of having a dedicated griddle however, I'm still researching a full size insert option but for now it will do.
Update: I finally had the time to take the Weber 7404 griddle out of the 420 Summit and try it on the Spirit 2. It fits front or rear but the heating rack will need to be removed if placed in the rear as it could get hot back there and cause a burn to the hand while cooking.
Yah those warming racks are always in the way. Griddle or no griddle. Toss it. Or just put it on a shelf in the garage for later. Then 20 years from now, it will still look like new since you likely will never put it back in like most people do.
Update: I finally had the time to take the Weber 7404 griddle out of the 420 Summit and try it on the Spirit 2. It fits front or rear but the heating rack will need to be removed if placed in the rear as it could get hot back there and cause a burn to the hand while cooking.
Yes, it just sits on top of the grates and can be moved around. On the Summit it can only be vertically installed but it can be placed right or left. I was originally going to pick up a Spirit E-310 which they had available which would have allowed for a full griddle plate but for the cost of less than tax I grabbed this E-210..