One way to make an accurate fit between two odd shaped objects is to use scrapers. A very thin layer of color is applied to one object. The two objects are pressed together, transferring a bit of color to the other object where they touch. The color on the second object is removed by scraping, filing or other methods, also removing a tiny bit of material from beneath the color. This is the process known as blue and scrape, which is capable of high precision in metalwork. I used a variation of this process to seat the infills. For color I used carbon soot from a candle flame; the original carbon black.
Trim the candle for a long wick. It’ll make a long smoky flame. Play that over the casting to leave a layer of soot in the area you’re working.
Press the infill carefully in place. No wiggling.
Pull it back out and inspect the contact surface. See the black spots?
Scrape them off.
Repeat. It’s a slow and exacting process, but it gets you there.