I like the Jasper jigs if the circle that I'm making is a dimensional increment that matches one of the incremental holes of the jig. If not, I have been using my CRB7 jig. Harry Sinclair has tutorials on making a jig similar to a CRB7 if you don't want to spend the money for a ready made CRB7, but the CRB7 with it's accessories has the ability to make larger circles than Harry's jig.
There are a lot of ways to make circles with a router. The purchased jigs can make it easier if you will be making a lot of circles, but a home made version can be used if you don't see the need for spending money on one of the manufactured jigs or don't see the need for doing this very many times in the future. The very simplest jig can be made from a 4-6" wide piece of 1/4" plywood with the router attached to one end, with a large clearance hole for the router bit and a nail or screw driven through the plywood the correct distance from the router bit to match the radius of the circle that you want to make. You drive this nail or screw into the center of where you want to cut the circle, but don't drive it all the way in. It will provide the center pivot point. The router can then be used to cut an arc or circle in your project. If the material being cut is thick, make several passes at increasing depths of about 1/4" per pass until you cut through the material.
To keep the bit from cutting into the workbench I always place a piece of foam insulation under the work and set the router bit length long enough to cut through the work and slightly into the insulation board. My insulation board has been used many times and has grooves running in all directions, but none are more than about 1/4" deep, so I'm able to re-use it again and again for this purpose. A scrap sheet of plywood can be used if you don't have the foam material, or you can find a way to space your work above your bench with blocks of wood, etc. But you will need to keep these spaced away from the hole you will be cutting. I prefer the foam.
Central North Carolina