According to Gwynedd Archaeological Trust, there are the remains of a cairn and cist right on the summit of Diffwys:
On the highest point at 750m, approx 18m N/S and 12m E/W, with high crags dropping to the E.
Heavily robbed for the modern wall crossing the cairn but with undisturbed cairn material surviving up to 0.5m above the natural hill top.
Just W of the wall are two arcs of large, radially set stones and two large edge set stones, 1.5m and 1m long, set at 90 degrees, forming part of a cist which appears to have been robbed.
Kicking myself that I didn’t check Historic Wales before traipsing up here in September!