You really need a small/square oil cooler behind the foglight to allow it to be mounted perpendicular to the incoming air flow for the cooler to be most effective (so that air flows through it).
Currently the air flow will be going around your cooler and not through it since it will take the path of least resistance.
I found the Setrab STD Narrow 25-row Oil Cooler (L:210mm, H:193mm, D:50mm) fitted well (see http://mx5cartalk.com/forum/viewtopic.php?f=57&t=41091&start=161)
I added ducting to mine too to force all air through the cooler and not around it - this would definitely help your cooling too since it creates a high pressure area behind the foglight and will force the air to go through your cooler since it can't go around it.