Since the original post was never updated to include the correct information I'm going to put it all together here.
There are three possible locations for the portal leading to the Skeleton Garamol rare. All three are within the Subterrane - Central explorer area. Icy Rainments and various random named items drop from this chest.
To find the correct entrance follow the backwards gold "G" symbols found in the Giantish Runes. There will be one at every junction along the route*.
For the most part it's as simple as that, follow the Gs and you'll get there every time. To make things a little easier when doing loot runs I favour the route below (thanks to Da5h-au for the base map)
The gold Gs indicate the two places you need to be looking out for signposts.
The crossed out G indicates a signpost to ignore. Ignoring this one avoids a couple of hold ups at doors/portals.
* Runes without a G indicate the portal is straight ahead or behind you, if you follow the map you'll only encounter this at the final signpost.