If you drilled two holes the size and spacing of your pipe bed in a block then cut through the centers, then measure from the bottom of the pipes to the top of the bench, and cut the block that length, then drill longways through the part above and below the bed for a bolt, that whole system would keep the rails from spreading and hold it still.
A coupling nut glued inside a wood knob would make it easy to place and remove.
Coupling nut: http://img0.fastenal.com/productimages/1170945.jpg
See sketch below.