That photo of reflextix stuck up in a vent is not a Heki. The materials the dome and frame are made from on that one are much different, more heat resistant, and much cheaper to replace than the Heki. I wouldn't chance that with a Heki. If you do, monitor the temperatures inside the double pane dome and report back. Reflextix over the top would be another matter.
For others that don't own a Heki: this is a European made vent hatch that has very different construction from the usual vent or over-the-bed escape hatch made here in North America. It is a great idea, poorly executed out of unsuitable materials. It costs many times what an ordinary one costs. You cannot treat it the same as the normal RV roof hatch.