Rebel, since there are holes in the bottom as well as the top, I can see where adding a fan, to an incubator that didnt come with a fan, could cause heat and humidity problems. Mostlikely the fan is also produceing heat. In that situation, you are most likely correct that placeing the fan over, or to close to, the top vent holes will cause excessive air exchange. Moveing the fan or resticting the top vent hole would be the best option. Even still, plugging the top vent hole completely wouldnt be my ideal remedy as that would stop the air exchange altogether. My best guess as to why people have so much trouble adding computer fans probably has to do with excessive fan speed and air circulation. Since a computer fan is normally 12vdc, using a phone adapter that only puts out 5vdc would make a logical choice of power supply and should reduce the fan speed by half. Provided of coure the PC fan being used will operate off of a variable voltage range of power. Some will operate at voltages as low as 3 or 4 vdc and as high as 18vdc, and some wont. I just purchased two computer fans that run off 115vac to use in a cabinet incubator. One will produce 110cfm's and the other 198cfm's of wind. Either of these fans would blow the styrofoam incubators off the table. LOL If I was to purchase a computer fan for one of the LG type incubators, I would probably look for the smallest one I could fine and then run it off a lower voltage to reduce wind speed and reduce heat buildup caused by the fan. As I said, you are only trying to create air movement, not a wind or breeze. Even hot air blowing across an egg will produce a cooling effect, this due to the evaporation of moisture from inside the egg.
Good discussion, keep it going.
Bill