I have seen routers mounted with 3 toggle clamps, screws going down from the top into the base, and even held in place by a hydraulic jack. (I wouldn't recommend the last one) So whatever you can think of that will be safe and meet your needs for depth of cut and ease of access. You will have to change bits once in a while so screwing it to the top will be cumbersome at best. You could make yourself an insert if you want to go for the least expense. There are many ways to solve you problem, it just depends on what you want.