The best time for these cases was 500 yrs ago, in an era of relics. There would be three different heads of the same saint, all equally authentic and miracle working, in three different places. So for this funeral box, too: there would have already been several of them, and finding or making one more would not present any problem; the first miracle attributed to it would immediately authenticate it beyond any doubt.
Five hundred years ago, there was no technology to authenticate these objects, so it was merely a matter of convincing a few powerful people. The people who made the fake James ossuary should have known that anything even approaching the importance of this would be examined by the top experts in the world and eventually the fraud would be discovered.