Well a cooler should have access to plenty of cool air flow mate, so it's not something that you'd want to place behind a hot engine, and you definitely don't want to use it as a fender, rocks would damage it and make it leak, and mud would block the cooling fins, and thus, the air flow. The best place for an oil cooler is on the front of your bike.
I guess you could build an oil tank, plenty of folk have already, I always thought it'd be nice to have one incorporated in the "seat hump" of a race bike, and I bet there are plenty of folk around with the skills to fab one up for you. The only problem might be that if not well insulated, you might burn your butt. You could build an oil tank incorporated into an alloy race tank, (as was used in some early US and British bikes) but I don't know if having a tank full of hot oil in close proximity to a tank full of cool fuel and some rather sensitive body parts would be a real good idea. Cheers, Terry.