Next step will be to epoxy the rest of the centering rings on.  For this build, since the motor mount tube to airframe wall gap is so narrow, and the fins are so long, it would be very difficult to get a good internal fillet working from the bottom of the airframe as I normally do.  Instead, I will install all 3 centering rings on the motor tube, then epoxy the motor mount in place.  To get proper strength for fin attachment points, I will drill several 1/4" holes along the root edge of the each fin, on both sides, and inject epoxy through the holes.  This will essentially "case" the fin joint both at the motor tube and airframe tube.  This step is probably overkill since I will be laminating 2 layers of carbon cloth tip to tip over the fins, but solid fin joints are never a bad idea.

While fitting the motor tube and getting everything properly aligned, I usually install, remove and reinstall a dozen times.  To make removing the centering ring easier, I decided to mount the Aeropack motor retainer first.  I'll just say this, Aeropacks are awesome.  Lightweight, clean design, nice machining, bombproof and easy to use.  Best motor retainers available if you ask me.  This is my first experince with the flanged version, and I've decided they went with 12 screws just to give me every possible opportunity to screw up the install.  I am sure after cranking down on these screws that 4 would hold it just fine, the other 8 are just to make me suffer. For the record I actually flirted with doing it right...

The first picture shows the "jig" I used to center the retainer over the motor mount tube.  I had to use a piece of PML phenolic coupler since I don't have any 98mm motor casings.  Once I had it perfectly centered and taped tight enough that I was confident it wouldn't move, I took a hammer and awl and punched marks in the center of each of the 12 holes.  I then drilled one hole to be the guide, as shown in the picture.  Each subsequent hole I slid the retainer back over the centering ring, checked to make sure my punch mark was centered, then drilled another.  I swear I must have installed and unistalled the screws 20 times checking to make sure the retainer stayed perfectly centered over the tube and all the holes fit.  Each hole was tapped with an 8-32 tap and the 11 screws went in smooth as butter.  Almost perfect... The last screw somehow I just barely missed.  After enlarging the hole just a bit I managed to get it in, but tonight I will probably fill that hole with epoxy and fiberglass so I can redrill it perfectly.  Even the that hole still held the screw pretty tight, but it bugs me, so a fill and redrill is gonna happen.