Using sand paper on a stick, or more specifically a spare leg for the mid-power launch pads, I sanded the inside of the body tube to prep for installing the motor mount.

I don't have any pictures specifically of the motor mount being installed, but you all know what that looks like, so we'll just assume that it was done. Once the mount is in place, I could install the fins. I used Dave's method of spreading epoxy over the slot and pushing the fin through. This seems to do a decent job approximating internal fillets. One side effect of the extended fin slots was that once the first one was in place, I could drop the other two fins in their slots, then push them forward into the fin jig, so the jig never had to pulled back during the process.

Then, I just turned it over every few minutes to attempt to even out any flow as the epoxy set. You can sort of see the filletation:

Now that the fins are attached, the rear centering ring can be installed. In this photo, the boat tail is re-dry-fitted.

You can see that the cone shoulder and the centering ring block part of the slot, but there's a half inch or so between that will need plugging. To do that, I placed pieces of masking tape to cover the inside of the open slot:

This should suffice to hold the epoxy filler while it sets, which will close the gap and get smoothed out on the outside. Nextly, the tail cone itself has to be installed. I haven't glued it in to this point yet, because although I intend for it to accomodate an Aero Pack 29mm retainer, I didn't have available on hand to actually measure. I used the dimensions published on Apogee's website for it, with the hope that they were accurate. By holding off on installing the part, I have the option of having a friend stick it on a lathe and open up the bore if needed...