Too late for fin can pics, already glued into airframe.Not even sure what that means
Right on ! I use that method also but I have found it difficult when the space between the airframe and the motor mount is restricted.A twist on the Binder instructions that a lot of builders use is to glue the forward centering ring to the motor mount, leaving the rear centering ring unglued with a couple of screw eyes mounted facing aft. Tie strings to the screw eyes, then glue only the forward centering ring and mount into the body. The unglued rear ring centers the mount while the glue on the forward ring dries. Then remove the rear ring with the strings and add internal fillets. Remove the screw eyes and strings and glue the rear centering ring to the mount and add fillets. No repairs needed to the slots.