What to do?

The best way to build a rail, if that's what you mean is to buy an equally long 2 by 4 and mount the pvc pipe on there. You can do this by drilling up through the 2x4 into the bottom of the pipe so it stays on there. Then, attach a 2x8 or 2x10 on to the end perpendicular to the 2x4, that being the 'feet' of the rail. Then make it sturdy by cutting parts of another 2x4 and screwing it onto the sides on the pipe so the pipe won't move around. Sorta like this



thats for a rail rather low to the ground

Instead of a U shape, cut the bottom board using the inside measurement to the two side walls. So you can lower the walls. This way you have more pipe exposed to slide on. Cuz the looks of that pic, it seems like you have to be rigth on top of it to slide it.