I got the griddle hack and made some smash burgers today. It fit my 780 perfectly, and heated up like a champ. What you do is take the grate, grease pan, and heat deflector out and this rests in place. I should have taken the top rack out too, but I forgot. I started the grill up, set to 275. Since you do the cooking with the grill open, I left it closed until it got up to temp. I didn't want it to fault out for not reaching temp after a long time because the cover was open. Once it got up to 275 I opened the cover and let it get even hotter. Since the probe will show less than 275 with the cover open, I think the grill basically goes to full auger trying to get caught up. The griddle eventually settled out around 450.
I didn't get any pics of the completed burgers, but they were fantasic!