Sorry to come in late on this one. Tooling is going to be the key here both because of the slot depth, material and machine. I would look into what Niagara Cutter has to offer that might be of help in cutting this slot. They have lots of specialty aluminum cutters for CNC machines designed to lower cutting pressure and increase feed speeds. This is really what you need to do here. I would give them a call and see what they have. And NO I am not of the the several dozen YouTube creators that received boxes of tooling from Niagara Seco. I have nothing against those that did, I just want it known I am recommending them based off cutters I have purchased from them for some specialty applications; not because I have been gifted tooling from them.