If the machine you are making the parts on is set up correctly then milling out the slot and the parts should not be a problem and should give you a straight edge to the horizontal plate and a nice flat slot. When bolted up the flat faces should meet nicely at 90deg. 2KG is not a heavy load on the plate and I would not worry about that bending the plate. If this is for a mill or router then I would worry about the forces the cutting process would exert on the joint. I think these would be more likely to deflect the joint. If this is to hold a spindle then go for two supports.