Thanks! All good points. I have a bit of slop in my runners, so I need to move them out to hit the edges of the channels as you describe.

I did secure the fence that way too, but I'd love to have some kind of very fine-pitched screw or something to act as a worm gear to move one end forward or backward. But really... I'm just too lazy to fix it at the moment!

I looked for some very thin, flexible HDPE sheet to use for another project, but haven't been able to find any. Seems like that would be a good coating for the underside, doesn't it? If you could find a way to attach it with countersunk screws or something, since I don't expect any adhesive to work on it.