Rail Jack: The base of my rail jack is a length of 2x6 attached to a 10-inch-by-14-inch scrap of 1/2-inch plywood. The upright is a 37-inch-long 1x4 screwed to a 22 1/2-inch-long 2x4.
A pair of slots cut into the upper half of the 1x4 allow a second 2x4, 9 1/2 inches long, to slide up and down on three screws.
Height adjustments are made with a 1/4-inch threaded rod and a T-nut. The 20-inch length of threaded rod passes through a hole bored lengthwise in the upper 2x4, through the T-nut, and several inches into the lower 2x4, where it bottoms out on a 1/4-inch ball bearing. Turning the handle epoxied to the top moves the upper 2x4 up or down.