Mine seem to show a preference for well-dried pampas grass, which is both soft and pliable. Soft well-dried grass or teased strands of hessian, cut to manageable lengths are preferred as well. The height of positioning the nest in the aviary, I can only speak from my own experience is between 1.5 to 1.8 meters. But again that's not to say, having found a nest box they like at a lower level, that they won't accept that, and happily set up home in it. Between four and six eggs seem to constitute the lay, both parents sharing the incubation, which they do with tremendous dedication.
Red faces are highly tolerant to nest inspection.
But having said that, I still suggest that you treat this tolerance as a privilege, and don't abuse it. As a result of your nest inspection, should you find a chick or chicks with a skin color that is darker than you would normally expect, this may indicate de-hydration. This may suggest that you need to check your breeding diet, and may call for additional green feed, or live food, such as mealworms or maggots. Contrary to what I have read from other breeders, regarding the pair ceasing to brood at ten days, I have not found this to be the case. In fact to the contrary, I have found that they will brood up until a couple of days of fledging.