Check here for one example of the cooler in the back:
http://www.pelicanparts.com/techarticles/914_ext_oil_cooler/914_ext_oil_cooler.htm
Agreed that the front is the "best" in terms of actual cooling. But you need some ducting or a shroud to make it work effectively, otherwise the air mostly goes around the cooler.
The engine-lid version is proably the least-effort solution; the front trunk the highest-effort one. Mine is going under the rear trunk floor when I get it in; I feel that is a pretty good compromise.
--DD