A simple parametric wrench, for when you find that you really could use a 5.5 mm wrench, and don't have one. (An M3 nut is 5.5mm.)

Edit the "size" variable at the top, given in mm across flats. If you want an imperial size, just enter, say, 1 * 25.4 for a one inch wrench (quite large). Depending on taste and how hard your plastic is, you may wish to edit the height, bar_length, and wall_size, the defaults are suitable for my 5.5mm wrench, ABS, and not terribly much torque.