You may want to consider something to put in place of your bottom grates. I can’t remember where I’ve seen the picture but the person had the grates out AND the drip pan out. The heat (fire pot) deflector was kept in place. And the griddle appeared to take up most, if not all, of the space the bottom grates (shelf) took up.
I imagine this setup allowed the griddle to get hotter with a bit more direct heat without just having the fire pot heat the griddle directly.
Another thing you may want to consider if you move forward with this pursuit is “whatchya gonna do with the grease”? Most griddles have a drain hole…that drains to a receptacle. With the backyard beast, you’ll need to have a griddle drain hole that would channel grease to the Beast’s regular grease drain/trough. You certainly don’t want grease draining into the bottom of the cooking chamber/barrel. Grease fire risk.
I guess you could keep the grill’s drip pan in place. But, depending on what you’re cooking, a grease fire on top of the drip pan but under the griddle could be problematic…and unsafe.
And how do you plan to control the heat on a griddle in The Beast? The PID controller? I don’t think so. If you should leave your grill’s lid open, which would be expected if cooking on a griddle, the cooking chamber temp cannot achieve whatever temperature setting you set when you first powered on…even a low 180 or 200F. The heat is escaping past the open lid. The controller will do its job by attempting to make the fire hotter and hotter to achieve the set temperature. But the lid is open so it never achieves the set temperature but keeps trying. After some time, you’ll have a roaring fire in the fire pot. That’s my hypothesis of what will happen. It’s the logic of how the PID controller works. It may not be a bad thing (that the fire pot is roaring hot) but I just don’t see how you’ll control the heat to the griddle.
Alternatively, the heat from the griddle could affect the temperature reading from the grill’s internal (ambient) temperature probe. That could impact how the controller responds by feeding the fire or reducing the feed if it detects the chamber is too hot.
Those are the things that came to mind pondering how to solve your griddle objective. Good luck.