a well built engine will not need an external oil cooler. rarther than worrying about such additions, try ensuring the hot & cold zones are well sealed, use good oil, ensure the engine isn't under too much load, make sure the oil routes and pump are in perfect working order. maximum you can do is get a vented boot lud or in some extreme cases you can fit standoffs but honestly they are more of a precaution.
at one time i was thinking about going external too but i've hammered my 1303 and 182 many times in summer and found that they are fine as long as i take some precautions (as above) and by opening the engine lid when i arrive at my destination.
fitting an external oil cooler is simple but it requires a bypass, many people make them but i'd not suggest a particular one. whatever you do, do not go for a local conversion, invest in a high quality item which is designed for your car. also do not fit it up front, there are many places you can fit the cooler in the back of on a VW and still get plenty of cool air to it (maybe a duct would be helpful too)
