Unscrew the rail from the base, we will be using it in this step.
Using a 1 1/2" circler hole blade cut a hole in a 3/4" thick piece of wood. Take out the wood (see photo) from the blade. This will be used as a roller. Make four of these rollers. A "5D 2 inch common" nail will become the shaft of the roller.
Cut two 3/4" thick pieces of wood 3 1/2" by 5 7/16". Place a roller near the back top of the wood but do not let the roller go pass the edge of the wood. Hammer the nail in the center of the roller but not too far in because you will be pulling them out later. Place the rail on the wood, make sure the rail is straight and tight against the 1st roller (see photo). Place the rest of the rollers tight against the rail and nail in place.
Nail the four 5D common nails into the four holes in one piece of wood. Now you want to drill four pilot holes in the path of the rail. Screw in four # 6 drywall screws but leave the heads about 2/16" above the wood. You will need to adjust them later. Put the four rollers and rail back in place. The four screws will let you adjust for any bow in the wood. Adjust the four screws so that the rail is even with the top of the four rollers. The screws may be needed to be adjusted again later. Take the 2nd piece of wood and hammer it in place over the 1st piece of wood. You will have to play with the 2nd piece of wood a little until it is close to the rollers but yet the rollers can spin freely. You may also have to adjust the four screws so that the rail is not too tight against the side of the wood. Slide out the rail.
Cut another 3/4" thick piece of wood 3 1/2" by 9". Nail the roller section to the top middle of this piece of wood. Cut another piece of wood (about 3 1/2" by 2 1/4") that will cover the other side of the roller section and nail in place. Now cut off all the screw and nail ends that are sticking out.
Slide the rail back into the roller section. There will be spots that will bind against either the rollers or the screw heads. Using sandpaper (best if the sandpaper is mounted to a block of wood) sand down the spots on the wooden rail where it is binding.