I use the inverted ash tray as a baffle in my CG, but have not found the need to cut or drill it. However, your cooking conditions may vary from mine, so what works for me may not work for you. However, you may want to give my method below a try before you cut or drill.
I take the cooking grates that came with the SFB & place in the bottom of the main chamber. When you set the inverted ash tray on top of theses grates, you can cover the SFB opening completely. If I push the tray flush & cover the opening, the stack side of the MC will run from 15*-20* hotter than the SFB side. There is an inch or so gap between the edge of the ash tray & the front & back walls of the MC that allows heat & smoke to rise thru the middle of the MC. By sliding the tray away from the SFB opening slightly, I can tune the temp difference from end to end to within 5*-10*.
With the setup, I can slide the tray further away from the opening & create a hot zone on the SFB side when I want to. I often use this for crisping the skin on BBCs & wings or glazing sauce on ribs, & so forth.