the best option i've found for a flat roof is 18mm shuttering ply on 2x2 battons running front to back of the loft, this will give you a 2" air gap front and back and then cover with the corugated bituman sheets. The wood will prevent condensation and help keep the loft at a more constant temp.
If you realy wanted to you could a couple of 2' squares, fill those with twin wall and then put clear corugated over the top, this would prevent condensation but allow extra light with a sliding shutter if you wanted to control the light.