This is a minimalistic fan guard for 40 mm muffin fans. It does fit the extruder fan on the Replicator 2's if for some reason you want to replace the metal fan guard.
For a cool looking, if slightly absurd, fan guard: http://www.thingiverse.com/thing:70981
I printed this in PLA on a Rep2. 100% infill, 0 extra shells, 0.2 mm layers, no raft or supports.
The Solidworks source file is included.