A small piece used to mount a 4cm fan (3.2cmm across from center of mounting holes) Used to keep prints cool as they are in progress and to keep the fan from colliding with the vertical bars and print in progress. The fan I use is a 12V small fan that came with my Makergear hotend kit.
Print the piece. If you're printing with a Rostock, make sure that it doesn't warp (Because that's what this piece will be used for ;3) and then drill out the mounting hole if necessary. See pictures attached for where to mount fan. The fan wires are connected to D9 on RAMPS. If you want to be able to control the fan directly from Pronterface, use M106 S(0-255) to activate with power S (255 is full power) and M107 to turn it off. You can also enable cooling in Slic3r's Filament Settings>Cooling.