I attached piano hinge to the seat back at the bottom, so I can access the back really easy. While that was curing, I glued in four 5/8" spruce braces for the seat back, to make sure they won't buckle under load. I also cut a 1/4" piece of spruce for the other side of the seat back hinge to attach to.
Once those had set, I cut the seat pan to fit around them, then connected the seat back to the 1/4" spruce with the other half of the hinge. These I lined up with the seat pan in the plane, then epoxied the spruce hinge plate to the seat pan.
Once that had cured, I drilled holes all the way through the seat pan and the spruce hinge plate and epoxied nuts underneath the seat pan, so the hinge can easily be unscrewed from the seat pan. Honestly, that was probably overkill, because it's easiest to pop the whole seat assembly out of the plane and pull the hinge pins. However, it makes me feel better that the hinge is screwed down instead of just epoxied on, and even if the epoxy holding the spruce hinge plate failed, the screws go through that *and* the seat pan, so it's not coming apart unless I want it to. I have a close-up of the hinge and spruce, but it's too blurry to post, so I'll have to take another one.
I went ahead and cut the seat back into two pieces after this was all cured. I didn't cut it before because I wanted to make sure both seat backs were hinged in exactly the same place and direction, so they won't interfere with each other. The last thing I did was seal the edges using 1" fiberglass tape, then sanded them all smooth. I'll probably paint the seat back, but I didn't feel the need to do that yet. With the seats hinging separately, I can use the passenger side as a flat surface when I'm flying solo. The only part that I haven't figured out yet is where to put the seat foam if I do that... but I'm hoping I won't have a reason to need a work surface while I fly. Much of my cross country flying has been done with an instructor in the right seat, which leaves no room for charts in a C172. As a result, I've got a nice kneeboard that will hold a folded chart. I'll be running iFly software on a Nexus 7 on the instrument panel, so in theory the only time I'd need a physical chart would be if my electronics went south, and I probably wouldn't bother moving seat foam and flipping the seat back down in that case.