While I absolutely agree that the Sith dagger in RoS is the epitome of silly MacGuffin-hood (Why does SW need a MacGuffin, JJ? The OT got by just fine by pushing our characters to locate other characters as an inciting incident and then letting the action happen.), the dagger's shape isn't actually THAT entirely far fetched. I still don't think that being able to potentially explain away the level of happenstance required to make it work justifies the laziness of it, and it certainly doesn't excuse any of the other logical leaps necessary for that branch of RoS's plot. But I think if you really wanted to you could explain it away.
Consider, Palpatine's whole thing is his nearly unrivaled foresight. More than any other power he possesses, it's that gift for making the future less "in motion" and more "going my way" that makes him a threat. Well, right up until he misses the big Anakin Skywalker shaped elephant in his throne room, but figure the Matrix rules are in effect and he can't see past a decision he doesn't understand. In that case, Luke's refusal to turn to the dark side. But, other than the big finish in RotJ and again in RoS, the Emperor is a man of almost uncanny ability to read the tea leaves.
I don't think it's an entirely ridiculous notion, given that Palpatine's example already exists, that the maker of the dagger simply saw a single moment accurately enough to create the precise shape required from that exact spot in that particular moment.