Wern Isaf above llangollen probably fits the bill. I walked past it last w/end and it looked like there was space. Station camping further towards Corwen is OK, but I bet its booked up by now, but if you don't ask.....
The ones down near the Abbey in Llangollen used to be pretty good I think - but ages (over 20 years!) since I stayed there, so things may well be very different now. Also Wern Isaf I think was pretty good.