I recently did some experiments trying a variety of methods to attach .030 Evergreen sheet fins to a BT-20 tube. Ended up with Testors red-tube cement for the initial glue and the fillets. Slightly roughen the glassine with 220 grit where the glue will be. I first glue the fin to the cardboard with just enough glue to form a bead on the edge. Then once that's dried nice and solid (evening to morning or morning to evening), I mask the edges of the fillets with the 3M purple "delicate surfaces" masking tape and pull them with a flat blade mini-screwdriver. The glue shrinks and they become nicely radiused in one shot - no repeated layers on top of layers. The surface is not quite as smooth as a good epoxy fillet, but by far the best adhesion of anything I tried.
I basically do one fin per day, so your build schedule will be as many days as fins, assuming your fin fixture does one at a time.
Once it's complete, it needs to sit around for a week to a week and a half for the solvents to work their way out of the polystyrene, otherwise the plastic is compromised in and near the fillet. The glue will be nice and solid after a day, but as you apply force, the polystyrene will bend easily adjacent to the fillet due to having absorbed the solvents. Once the solvents have evaporated for a week and a half, though, it is far more robust than any of the other methods I tried. So just get it done, then put a "Don't F with it" field around it for awhile.
Other things I tried:
E6000 - Stayed flexible when dried. That flexibility allowed it to concentrate force and peel the fillet off the cardboard. It also peeled fairly easily off the fin, although it did leave behind a disturbed/solvented surface.
Duco cement - In spite of being a heavy solvent glue, it came unstuck from the polystyrene like there was mold release on it. Virtually no chemical interaction/bonding between the glue and plastic. Also, difficult to find reasonably priced. I noticed that where I'd put some on some office paper, it continued shrinking for several days, just like you'd expect something that's basically really dense butyrate dope to do.
CA with epoxy fillets - The epoxy peeled off the CA easily, leaving the brittle CA to fail easily.
