OK. so the upper ID of the Booster has been sanded to fit the lower end of the av-bay. Still not as smooth as I'd like so a bit more fitting will be done tonight. The top cut of the Booster has been sanded level to match up with the vent band on the Av-bay.
The lower CR has not been epoxied in place yet, but the aeropack has been installed. Lacking a 76mm motor, I slid a very tightly fitting 3" FG coupler into place. Drilling holes and installing screws one at a time, opposite of each other until complete. There are currently no backing nuts. My current plan is to bury it all in epoxy.

I have not yet finalized my plan on installing everything. Working out the locations of the CR's and epoxy order. Putting the rear CR up to the fin tabs would be ideal, yet complicated. If I leave the tabs above the bottom CR it's a nice and easy glue it up, then butter up each fin to tack in place and then 3 rotations of injections. Of course, If I add a 4th CR, I could have it both ways. (or use the middle CR just above the screws, and below the fin tabs. This would leave the front of the fin tabs not touching a CR. Tentatively, I like this plan the best. Aeropoxy doesn't run out too quickly, and I could always make a forward dam out of tape wrapped around the MMT.) Thinking out loud there.... And I think I like that. Makes for a solid setup and many long strong fillets holding the fins in.