I like the Payload Bay centering ring tool. I would just use 3" as the outer diameter and 24mm as the inner diameter. Print that up on regular copy paper. Find some cardboard in your recycle bin. Glue stick the template onto the cardboard and cut out. You may need to sand down the edges, or cut slightly inside the lines to fit the inner diameter of the 3" O.D. body tube. Likewise cut the inner hole a little beyond the 24mm to get it to fit the OD of the engine mount tube. The Big Daddy fins are TTW, so the cardboard centering rings should have plenty of help holding those D and E engines in place, but if you think it needs to be stronger, glue stick another layer of paper to the other side, or laminate two layers of cardboard with the corrugations at 90-deg angles. You can probably use the cardboard box that Estes used to ship you that body tube. Make the two CRs at the same time, and sand both simultaneously to fit like you would a fin stack, so that the engine mount will be on centerline and parallel to the main body tube. You can get it pretty accurate.