Two small welds broke on my bed frame causing a large sag in the mattress. This simple hack fixed it. It's been working fine for over a year. The plastic block on the frame is not noticeable. 

I give this fix 2 thumbs up. I was surprised that it worked so well and was so easy to install.

I used PETG. For extra strength I printed with 0.3 layer height, 40% infill, and with 5 horizontal/vertical shells. I printed it on it's back with supports. This first print worked fine, so I never tried any other slicer settings.

After printing I hammered the printed part onto the inner rail, because it was a tight fit. It then dropped in place over the outer rail.

Just incase it was needed, I added screws holes so that the block could be fastened to the outer rail, but it turned out that those were not needed. It works fine with no screws. Maybe that's a good thing, because the screw holes would weaken the outer rail.

The OpenSCAD source file is included.