The problem with this is the DM is generally against custom stuff, otherwise, this would be rather cool, albeit a little powerful. They don't mind custom magic items, so maybe they would be inclined to allow adamantine bracers of defence... Thanks for the idea!
The problem with this, and the reason I don't want to take it, is that it only has 3 charges per long rest, and that may not be enough. When they have been a player, I've seen the DM roll 20s quite often.
Unfortunately, it seems that taking the Lucky feat is my only option. Since the campaign is in the epic levels, I will have access to boons. If any of you are DMs, how would you rule using the Boon of Fate on a crit? Would you allow it to negate the crit or not? I am looking at this as a potential solution once per short rest, and then relying on Lucky after that.