Estes briefly sold a "fin alignment jig" - a little plastic device that would hold three or four fins in the correct positions around a body tube, so you could glue all the fins on at once. There were mixed reports on how well this worked, but surely the concept makes sense.
Here's a variation o the magnetic clamp adapted to a similar purpose. It's a bit more flexible, and it should adapt well to larger rockets. Likewise, it's probably easier to make mistakes due to the extra flexibility. It's easy to make and uses rather neat super magnets scavanged from old disk drives (or you can buy the magnets, if you want.)
