Little bit of work but you could do a servo under the carriage with some sheathed r/c plane throttle cable. Fab up a small bracket to hold the sheathing at the tube so just the cable would actuate the barrel. I did this on my resin KV-1 conversion, made a turret basket and mounted the servo in the basket, ran the cable to the bracket where you'd usually have a recoil unit. I had clearance issues so just having the small bracket to hold the cable sheathing gave me the room for elevation. Really though even a micro servo a bit forward and under the breach slide once painted would hide pretty well.