Good, cheap trolling motor/engine ideas

I have an 18' invader bowrider with a 150 mariner on it. I would like to do some trolling on the boat, but the mariner is certainly not made to idle for very long(it is the magnum high performance series) What do you think my best option for trolling is?
My dad has a 15hp outboard for his inflatable that he would probably let me use. Should I make a bracket to put that on my swim platform? I really don't want anything perminatly taking up space on there. Or should I look into a trolling plate or an electric motor.
Opinions please.