I'm away from the house right now but I think it was a block of "Delrin
http://www2.dupont.com/Plastics/en_US/P ... elrin.html" (high tensil strength along with high lubricity or low friction and wear). I got it off ebay in mostly the size I needed (most importantly the 1/2 or so inches thick, I could cut it into most any other size with a table saw). I just ran a saw kerf down the length and then cut it off for the right width. used a slide vise to get the basics of the slot for the guide.
As far as the installation I ground off the back side so I could have the stud separate. I talked to the local welding instructor and he didn't think that we could weld the studs back in without melting the block a little so we built up the backside about a 1/4 inch and turned them down to fit back in the holes of the bracket then pounded them over just like the rivits for the "rollers" If it starts to work loose I can then try to have them spot welded.
So far it has been about 3 months and no problems and they work great.
Nathan