You can build yourself a jig out of almost anything, (including FB), to keep the fin in exactly the correct place and properly aligned but this can be somewhat model specific. I 3D printed something I use on FB tails but it is for 5mm material only and only hold the fin vertical, (it does not do or maintain any alignment holding/setting).
If you wish to make your own jig just let me know. Whilst I remember a good jig can be used during transport to stop the tail members from being damaged/broken except for the severe impacts of course!
To be completely serious, I make a lot of one time use jigs out of dollar tree foam board as Hai-Lee suggested. Cut a slot the width of the vertical stab at a right angle to one side of a piece of foam board and you have a jig.
I use a plastic square with the corner cut out for setting it all up then 30 min epoxy and babysit sit it with the square until it sets up. If everything is prepped properly ahead of time the tail should stay squared up. I also use a magnetic build system and will sometimes use different jig setups depending on the plane being built. The babysitting method works ok on smaller planes and it does take 30 minutes of your time.