Advanced Router Help

I am trying to make beaded edging on a concave surface. I located some info on how to set up my router by drilling and tapping holes in the housing to give me the depth ofd cut that I want, and this works great with one problem and that is how to keep the bit level with the workpiece. I get the bead but it looks wobbly becuase I can't keep the router exactly level as I cut. My question is there some way to make a guide to keep the bit level on the flat surface as I cut the edge?