If it's in Den of Rancor coffer, it shouldn't make it that much harder. Den coffer is fairly easy to find, spawn points aren't too far apart and there are no locked doors or anything.
The harder part would be getting there (defeat Temple Guardian and paint painting), also you'll need sneak in addition to invis while camping. This means thfs might need to sub whm or rdm instead of NIN.
I think it's a good idea to change Astrals from oztroja to den, since no AF pieces are obtain from Den's coffer. This way all the RDMs, DRKs and BRDs don't have to suffer hours of coffer competition.