I cut the MicroMaxx motor mount out with my mini-Leatherman and then took the Dremel to the inner ring until an 18mm tube fit. The cuts are shown in the following photos.
The connection point for the lower fins were weakened so I added some supports cut from a scrap plastic model. I attached a fiber ring on the outside of a short 18mm tube. This fits in between the inner ring and the lower fins. I used 5-minute epoxy to fill the small space between the top ring and the motor tube. Fillets were also added to the point where the fins touch the motor tube. A 1/8" launch lug completed the mod.
Hurricane Bill allowing, this will fly on a C6-3 at this weekend's MDRA launch.