Got mine set up!
I had previously cut a hole for the button but it interfered with the turn signal, so I had to cut another slightly lower. I need to get a big washer to cover the hole, but perforated board was all I had.
I attached the relay to a bolt-hole that was already in the rail, using one of the stock coated bolts used all over the car. I sandwiched the grounds between the relay and the rail, and ran the wire to my switch through the same grommet as the +12v for my amp.