Locking a sliding table

[throbscottle]

I had this marvelous idea that I can make a sander which has a table with a fence on it which can slide in and out on drawer runners, so long pieces can then be passed across the sander at an adjustable fixed distance from it. (My best shot at getting anything flat at the moment) The drawer runners would keep it parallel.

What's a good way to lock the position of this slidey thing without distorting it?

Thanks in advance