Hm, yes there is Glaslyn, but again it's another needle in a gigantic haystack. Anyway, wouldn't it have been easier to dispose of a body in Llyn Clywedog (lake)? It's far steeper than Glaslyn, so suitably weighted it would go pretty deep quite quickly. That B road goes right past it so it's quite accessible. It would be pretty difficult to get rid of anything in Glaslyn, especially late at night. I wouldn't like to go walking in/around it during the day.
Glaslyn is quite difficult to get to, as you say. I ruined a good shirt there a few years ago while volunteering with the Montgomeryshire Wildlife Trust. We were cutting back the Rhododendron and making bonfires out of the stuff. I got singed. So did my shirt.