I suppose that an enlarger would be workable. Lowere your head yo the bottom, remove your lens and carrier, tape the meter so the cell is in the center of the beam. Take a reading at a constant time. Note the fstop. Put the diffuser in the beam and note the fstop. How much light was lost? I would guess about 1.7 to 1.8 stops. As long as the light is bright enough to be well within the sensitivity ranhe of the meter it should work. As an altenate you could use daylight tomorrow.