In preparation of gluing the outer rail on, I made about 40 little friction clamps out of plywood to keep the top of the two rails together since I don't have 50 spare clamps lying around. I probably spent too much time sanding them all but I'm glad I did since I was pushing them down with some force and I'll be using them in the future.
Gluing the outer rail on went much smoother than the inner rail. I test fitted it and cut it to length, then applied the glue in sections as I went along applying clamps. I did do a bad job of wiping off the squeezed out glue, but hopefully some sanding will take care of that.