Cooler exit diffuser
If I ever re-do my firewall, or build another one, I will do something kind of similar to what Dan did. I will mount the cooler at 90 degrees to the firewall, and then make an exit diffuser for it.
The diffuser can be fairly short and wide-angle and still produce a nice increase in airflow thru the cooler. (you can google "wide-angle diffuser" to find tables of allowable angle vs. length or area change)
The diffuser works by having a larger area at the plane of the exit where the flow rate and velocity is set by matching the exit static pressure to the surrounding ambient pressure. If that area is substantially larger than the area thru the cooler, then you get higher velocity through the cooler (like a venturi: less area, higher velocity). The key is to minimize flow separation in the diffuser, which is why there is a tradeoff of length vs diffuser angle.
My main reason for doing this would be so that I can reduce my cowl exit area. My current set-up cools very well, but if I throttle the cowl exit, the pressure in the lower plenum will increase, and the flow thru the oil cooler will be reduced. An exit diffuser can compensate for that, maintaining high flow rate with the higher exit pressure.