I found the PML Quasar very challenging
when it came to the internal fillets. My solution was to use a syringe
to inject the epoxy through holes drilled in the airframe close
to the fin slots.
The fins were tacked in place with CA and the aft centering ring
was installed to prevent the epoxy from leaking out the back.
Magic tape was placed over the holes to keep the epoxy from leaking
out. The holes are eventually hidden by the external fillets.
The syringes weren't cheap, so I reserve this technique for rockets
that specifically require it such as the PML
Quasar or my upscaled Quest Intruder, the SkyPirate,
which required getting epoxy about eighteen inches into a three