Real cribs often use a metal rod to slide up and down on. I purchased two 1/4" steel rods for this purpose, but realized that I had no way to bend such thick metal at the desired 90 degree angle, especially considering that it would have to be inserted through the holes in the wood prior to bending the second end.

So my next idea was to use 3/4" aluminum angle stock. I cut a groove in the wood and hoped it would slide freely on this. Unfortunately the vertical posts on the crib weren't perfectly straight and perpendicular to the floor, so while it would slide freely in some parts, it wouldn't in others. After I glued the dowels in it also caused slight warping of the boards which further contributed to the problem.