All you need to do is before you fight the grandmaster you go to your stash and check what you need. It’s not that hard. Exactly what I did
Way back when I beat Act 6 for the first time I was under the impression it was going to be an item you could open when you wanted especially given it’s for the rarest item in the game. I was the first in my alliance to do so. Had I known I was going to have to decide right than, I definitely would have wait to complete it. It’s not clear anywhere either hovering over it in rewards or in the forum post made be Kabam regarding rewards it was going to be automatic. If it had been made clear I wouldn’t have completed it when I did as I said. I think the best option is to let players decide when they open regardless of when you complete Act 6. At very least they should make it much more clear. I obviously know now and think most people are aware at this point but at the there was not a single other reward in the game that made you decide right then. I don’t think it was unreasonable to think you could keep it and open when you want. I would have waited to complete it in order to see how my shards were after obtaining much more and I was closer to completing a catalyst. I knew I had very little shards in all classes but would have rather seen what I was closest to completing first.