The problem with putting a screen over the top of your stack pipe is that these plug up with debris and choke your fire. At the time of this writing, the Concentric Exhaust System is the best way to address sparks. With Concentric Exhaust, the steam from your evaporator terminates through the same roof penetration as your exhaust. The steam then envelopes and helps extinguish the sparks.
Regardless of whether or not you have Concentric Exhaust, never run your evaporator without a fire extinguisher close at hand. Make sure all of your helpers know where the fire extinguisher is kept too. Never run your evaporator directly alongside your home or other structure. Sparks can carry in the wind and land on dry leaves or your firewood supply so be sure to keep awareness of what is happening around you. Also, never run your evaporator without first tethering/securing your stack pipe.