The NMR record (available through Coflein) describes Disgwylfa Fawr and the notorious canoe type things that were found inside it:
Cairn upon the summit of Disgwylfa Fawr, c.20m in diameter & 3.0m high: trenching, in 1937, reported a ring of slab-stones within the body of the mound, within which were found two dugout tree-trunks of oak, 2.4-2.7m & 1.08m long, these subsequently yielded radio-carbon datescentring on 1910BC & 1350BC respectively; within the smaller dugout were cremated human bones, a flint blade & a ceramic vessel, covered by an animal skin.
The author of this text is the venerable Stephen Briggs.