Many ways to do this. #1 Is this the main panel? If SEU is the wire needed. Two hots And a ground. If you have a meter main, or Breaker outside .Then panel is sub-panel and need 4 wire ser. Don not see why you can not  nipple through meter into panel.And remove the short studs in the way. Use pvc little easier to work with.  SEU is aloud and is done all the time with out conduit. Just have to watch length of wire. And you could sleeve it to protect or not . Make sure you bond the panel if a main, should come with bond screw, or you have to get ground bar to add to panel and not bond panel.White or neutral to bar  on panel .Bare grounds to added ground bar. Also check the ground rods .  and wire to them If this the main panel run water bond to it. If sub-panel water bond goes to meter main outside.Cover hole bay with plywood, screw on  so easy to add wires later And re,id the ground wire ,green tape or bare.