I came up with this simple way to dowel a rail in place between two fixed posts. Drill and dowel one end of the rail as you normally would. On the other end, drill the hole deeper and insert a spring before the dowel. Make the depth of the hole equal to the length of the dowel plus the length of the compressed spring.
Once it’s aligned with the hole in the post, the spring-loaded dowel will push its way home. I butter the dowel with hide glue before inserting it in the rail. I also drill a small vent hole in the bottom of the rail to prevent suction problems. Beveling the ends of the dowel allows the parts to go together easily.
Thomas Ehlers, Austin, TX