I can use a square hollow metal bar as the housing for the pillow block. Use any round bearing. You can even use the ball-bearing-less bearings which are the cheapest as compared with ball bearing types. You can drive a screw into the metal bar which makes contact with the bearing to secure the bearing within the metal bar housing. I saw a picture of this in itemamerica.com. Here is the picture. However, instead of using a plain hollow square metal bar as the housing, it uses a piece of aluminum extrusion that is precut. This setup would be the least expensive, fastest done, with most accuracy that I can think of. I'm sure there are cheaper ways but they would not be accurate. See the picture to see what I am talking about. The only problem is that it might not readily work with rails that support the round rod. But maybe there is a way around that.
Here is the picture found at:
http://home.mminternet.com/~georgeb/item_bearing2b.bmp
Maybe someone can include this in a message. It is a big file.