Thermal management of MOSFETS in TO-263 package
Just a fan blowing over the PCB won't be sufficient; a TO-263 doesn't have good thermal contact with the air surrounding it. I would solder this

 

heatsink over the FET. Like I said in another answer earlier today an SMD loses much more heat through conduction than through convection, but the heatsink has better convection loss thanks to its larger surface. The heatsink mounts over the SMD, not on it.  



Heat transfer will occur via the PCB's copper, so increase the TO-263's pad so that the heatsink shares it.