I agree with Doug, bondo should do it. Me, I'd more likely go with a mix of glue and sawdust, partly because I don't have any bondo.
When I made my last router table I made my own inserts. No issues after probably close to 15 years, or more.
And, instead of trying to rout the sides of the insert, I would probably have sanded them to fit.
"It ain't what you're told, it's what you know." - Granny Weatherwax
Some days, the supply of available curse words is insufficient to meet my demands.
Call me a craftsman, artisan, or artistic, and I will accept that. Call me an artist and you will likely get a quite rude comment in return. I am not a @#$%ing artist.