I just completed both parts of this hack, the flapper, and covering the screens. I'm truly surprised by the difference it makes. In my 2020, Custom Trail Boss, with 5.3 engine, it initially had almost no sound at idle. And now, I'm very impressed with the $15 I invested.
First things first, I tested it out after only opening the valve, and the sound was not impressive. To be honest, I almost stopped there. But I'm glad I didn't.
One thing I did a bit differently is how I solved for keeping the flapper open. The 2020 Silverado has a round valve which is difficult to keep open with a single clamp. So, instead I turned the valve clockwise (open), and inserted a finishing nail through the top of the valve, and down towards the ground through the bottom part of the spring. It's difficult to explain, but with the nail, you are basically using it as a lever to hold the top part of the valve against the bottom part of the valve while in the open position. This took a little bit of maneuvering, but in all, it only took about thirty seconds to get it wedged open. And while the valve is probably only about 90% open, and is still a drastic improvement over the baseline.
Also, as I don't believe I saw this anywhere else, if you have split exhaust, you'll have six screens you'll need to cover.