I've made collar bases for my trim router from masonite (clear plastic would be better). Easy enough to do since, as Nathan points out, some adjustability is necessary.
I like having the base-to-router screw holes oversized to allow for the centering to occur there versus a "sloppy" bushing hole. That bushing nut can be a tough reach to cinch up while installed on the router.
A hinge mortice bit (with shaft guide bearing) and a template can be used for inlay routing as well, but you are limited to typically 1/4" radii on the corners and a minimum depth depending on the template thickness. Greater depths can be achieved once the bearing can ride along a previous cuts wall.