@redir, that second picture didn't come through. I like your idea about just rounding off the plywood guide block. Finding components is nearly as difficult as it is justifying buying all the expensive jigs from StewMac and LMI.
That being said, I got something workable put together. I haven't tested it yet though. I went with a 1/4" x 2" tension rod with 1/4" (inner diameter) x 3/8" (out diameter) x 1.4" wide nylon washers. I need to figure out a way for it to stay put in that groove but otherwise everything is aligned. I'm going to try to drill a hole through the guide black to thread a 3" x 3/8" steel sheet metal screw through and into the slot in the tension rod of the dumbbell. If it doesn't work, I've always got more plywood and threaded inserts.
PS: pay no attention to the laminate trimmer I have mounted to it. It's a $30 cheapie from Harbor Freight. I'm on a shoestring budget. The jig cost me less than $20 to make. The router bits cost more.