Unless you plan on racing or towing, the car shouldn't need a trans cooler- even at 40C.
If you want to add one anyway for peace of mind, I would look at getting one of the linear models that is essentially a length of straight tubing with cooling fins brazed around it. The standard models that fit in front of the radiator work fine, but you need to run a lot of hose. With the linear model, you can mount them closer to the tranny- just be careful to hide it behind the front suspension crossmember. You don't want to wind up on the side of the road because of a piece of road debris!