Might work not 100% sure
Invite 1-3 people in your party since it increases the amount of "turrets" you can place. (+ The amount of zombies spawned) After you've put down all the turrets you can just kick them and the turrets will stay there as yours. I have not tried this yet but I'm pretty sure that the zombie spawns go back to normal since the game registers only you in your party. And now you have way too many turrets for comfort and a pro base :).