I need to begin by saying that he already bought 1/2" plywood and cut it to size *before* he called me about cutting the ledged holes.
So here's what I'm going to suggest:
1 - Glue a piece of 1/4" tempered hardboard to the topside of the plywood. 2 - Use a circle cutting jig and an up-cut spiral bit to cut the circular h oles through the now 3/4" panel. (He's planning on making a circle cutting jig for his router) 3 - Use a slot cutting bit, with the bearing riding on the plywood to creat e the ledge by cutting away the hardboard top.
A quick look shows that Amana makes a slot cutter that will cut a 1/4 inch "slot" (in this case, the ledge) to a depth of 3/8”. I'm sure there are other slot cuttin gs bits that will work.
4 - Cut the insert disks from 1/4” hardboard.
Does that sound like a workable plan? Have I chosen the right router bits f or the task?