The old Estes fin alignment guide is a tool that does a pretty good job, but it is out of production. Any that show up these days on ebay go for $40-50-60 (waaaay more than I would pay for it)
In the case of a rocket configured like the Commanche, if you get one set of fins well aligned, you can use them as a guide to get the others on. Use a light-weight straightedge (school ruler?) and clothespin-clamp it to the attached fin so it extends over the other stages. Use more clothespins to clamp and hold the other fins as you build the other stages. Rotate to the next fin position and repeat.
Other ways include using a pile of magazines, thin books, or just plain paper (you can use part of a ream and still use the paper later in your printer) to build up a level flat surface that is tall enough to match 'half' of the rocket body diameter. Actually the pile needs to be built up to 'half the diameter minus half the fin material thickness' but that is kind of splitting hairs here. Anyway, lay the fins flat on the surface with the fin root hanging off the edge of the pile, lay the body tube next to the fin root, glue and let dry. Rotate the finished fin to the opposite side and glue the second fin on the opposite side of the BT (obviously, this is for the case of a 4-fin design). Or, move your 'assembly area' to the edge of a table so that as you rotate the finished fins they are hanging off the side of the table as you continue working on the next fins. (Was that about as clear as mud?)