a (very) minor derivative of rickmellor's eponymous thing. I'm only posting my changed file, Insert (flipped over).stl.
Follow the instructions on rickmellor's excellent contribution: http://www.thingiverse.com/thing:29726
I simply flipped over the Insert.stl file from his thing so it would print better for me. His file appears to be working for other people, so this may not be necessary for you.
Note: I have a Printrbot LC Plus and this thing works perfectly for me. As far as I can tell, it should work perfectly on any Printrbot with a laser cut wood extruder.