To integrate the sustainer and booster fin cans, we first 3D printed aligners that would help us locate the position of the fillets on the fin can. We then fastened rods through each hole of the aligners such that the top aligner and bottom aligner were...aligned! We then inserted dummy fins (straight bars with the thickness of a real fin) in the gap between both holes in the aligners. We attached painter's tape on the side of the dummy fins that were flush against the fin can so that no epoxy from the fillets would make them stick. The dummy fins, as well as the aligners, allowed us to epoxy a fillet on each side of a dummy fin that were evenly spaced along the fin can and straight. After the fillets were epoxied, we then inserted screws into the holes (screws were 8-32, 5/8 in). Next, we epoxied the forward and rear transitions on top/on the bottom of the fillets. When the real fins were ready, we replaced the dummy fins with them. However, there was a slight issue while doing so: most of the real fins did not fit in the gap between a fillet pair, even though the dummy fins did. This was purely an issue with tolerance (the dummy fins were slightly less thick than they should've been, and the real fins were slightly more thick than they should've been), and ultimately we had to mill down the part of the fin that attached to the fillets by a couple thou to make them fit. For the future, we think that attaching painter's tape along the sides of the dummy fin would solve this issue, as it would make its thickness slightly larger – the real fins don't have to fit extremely tightly with the fillets before they're screwed in.
Note: the integration process for the booster and sustainer fin cans are identical.