You are not going to like how I do my chat. I am using a Chrome plugin called BetterTTV from https://nightdev.com/betterttv/ . They have chat box pop out and a darkening option that pretty much sets the chat box up for green screen in OBS.
So before I stream I have to open my twitch dashboard website, pop out the chat, toggle it to dark and set up the window capture in OBS. Kinda a pain, but it works for now.
Once I have it set up In OBS I select the chat box while in “edit scene” and hold the ALT key while resizing the window. This turns it into a crop instead of a resize. This is how I cut off the top and bottom of the chat box.
I’ll try and throw up some screen grabs later tonight. Hope this helps.