If you look at the Graig Fan Ddu area you’ll see a trig point spot 642, I have seen tents just off to the left here as you look south, that’s if you want to stay high. For something more sheltered with a stream close by, look at the map and see a footpath going from Pont Cwmyfedwen following the Taf Fechan, approx two thirds of the way along this at a junction is a secluded spot for camping, I have seen the occasional camper there but it’s a seldom used footpath so should be suitable for you.