Like other solutions have mentioned, place 6 turrets and use your own weapon (beam or ball). This solution is just a list of maps that have the easiest rooms to set up your car wash and some general tips.
Temple of Anubis - Defence.From point A looking toward the enemy spawn there is a gate they have to pass through. To the right there's a small room which allows them to flank, but inside there there's room for you and your turrets.
Hanamura - Defence.Here there's also a gate at point A where all players that can't climb/fly have to go through. Before the gate on the left there's a room where enemies usually only enter from one direction.
Volskaya Industries - Defence.Once again at point A there's a form of gate, with a truck in the middle. To the left and the right of the gateway there are rooms.
Numbani - Defence.At the first objective there is a hallway to the right (bottom floor) with a larger room with a health pack inside and at the end an entrance where the enemies are very likely to come through. Try to place them very high up so people won't notice them.
Watchpoint: Gibraltar - Defence.At the first checkpoint there's a room on the side which the enemy often uses to flank.
A bit after the second checkpoint there's 2 doors next to the big door the objective comes through. Keep in mind that the objective door is usually the main way for attackers to go into that area, but it closes as the objective moves past it.
General Tips- Try to do this at the start of the match, because it takes some time to place 6 turrets.
- Try to place turrets on walls away from the floor, because Junkrat's grenades or Phara's rockets could destroy them.
- Try to bait people into the room you're set up by using your
(default) attack. Since it travels slowly and enemies see where it's coming from.
- Try not to bait a Winston, since he wrecks turrets and requires little aim effort to do so