He's arrived at the idea of trying to shape the solid material of a chakra-possessing creature into the form of a seal, had the tools to do that, and the sensory techniques to verify whether chakra flows right through the resultant "blank"? That's about 95% of the insights needed. The only missing piece is the idea of trying this out on a variety of non-human bones; the moment he gets this idea, he goes on a chakra-beast hunt, comes back with a trove of weird materials (recall that we actually got the lead about the crystal-entity cave from him), and cracks the problem in a couple months.