The Booster

I then sanded the inside of sustainer tube about 30" up from its bottom, applied a ring of West System epoxy along this ring, and slid the partially assembled motor tube up into the sustainer from the bottom.  I stopped just before the lower centering ring passed the fin slots, poured more epoxy up into this space, and twisted the assembly the rest of the way pushing the bottoms of both tubes down on the floor to ensure they were flush. Next I threaded two screw eyes into one of the two remaining centering rings and slid this into the base of the sustainer to ensure that the motor tube cured parallel to the sustainer body. :

I then stood the assembly upright and poured in a layer of epoxy into the sustainer/upper centering ring/motor tube interface and allowed all to cure:

Next I ensured that the sustainer tube was horizontal and one fin slot was pointing 90˚ up using a spirit level.  I test fit a fin into this slot, applied epoxy to the fin root, inserted the fin, and taped it in place.  I also verified the fin was normal to the sustainer tube using a spirit level and allowed this fin to cure:

I then repeated this procedure three more times constantly checking fin angle using a spirit level.  After the fourth and final fin had cured I placed the assembly upright, mixed up a batch of epoxy, mixed in colloidal silica until I achieved a thin paste, and filled in the gaps at the tops of the fins and allowed them to cure: