Use a chopper and a mirror. Mount a toothed wheel (count the number of teeth and gaps) on a motor where you can control (and measure!) the RPMs. Aim your light (laser pointers are nice) through the chopper at a mirror far away (measure the distance!). Stand directly behind the setup, looking at the mirror through the chopper. You should see the laser spot reflected back at you. With the motor off, or going very slowly, the light goes through a gap, reflects and comes back through the same gap. Now start turning up the motor speed. At some point you won't see the reflected spot any more. The motor is spinning just fast enough that the gap moves away from the light beam before it gets back to you; the beam is blocked by the adjacent tooth. If you know the tooth spacing and the motor speed, then you can calculate the time interval between a gap and the adjacent tooth. Since you know the round-trip distance (from the wheel to the mirror and back again), you can calculate the speed of light. With this set up, you should be able to get a 10% measurement.