We just bought this one on Monday:
http://www.toysrus.com/product/index...ductId=2552662
Wow...what a difference it has made. DD always slept in between DH and I (which I have now learned is not the safest) and DH was not getting good sleep because he was afraid of rolling over on her. I wasn't getting good sleep because I was always tittering on the edge of the bed. It has really changed our lives...and I am only being slightly over-dramatic.

As for sliding in between the mattress and the rail, if you tighten the straps very tightly, I don't really see how that can happen. This rail was pretty cheap and easy to put together. I would say that this one would work for a king as well, we have a queen and there is only about a foot and a half space open at the foot of the bed. I think a standard king is the same length as a queen and a Cal King is 4" longer. We also chose the "extra long, extra tall" version.
*Added - We don't put DD down for naps on the bed, one, we only have the rail on one side of the bed and two, she's a crawler/walker and could easily stand and flip herself over the rail.
Good luck!
