Make a template that is 1/4 " bigger in each dimension than the hole in the big piece. Get an inlay cutting rub collar for your router. These have a bushing that goes on the rub collar that you run on the template. You cut your mortise with the bushing off and then you put it on to cut the inlay. It makes you a perfect fitting inlay for whatever shape you want to inlay. It's super fast and accurate.