Once our DD6 outgrew the Pack-n-Play, we requested bedrails (2) from Housekeeping. We put one on each side of the other double bed, and the piled extra pillows and the comforter along the bottom. She was completely enclosed, and we had no problems with her falling out.
She moves around a lot when she sleeps, but in a double-bed, she never even got near the edge! There were times that, if we were stopped for a single night at a Hampton Inn or something, we didn't bother with the bedrails and just piled pillows around her. She was fine.
We do have a RediBed that we take if we're in a DVC 1-Bdrm. That way we can put her to bed in the Master and not have to share the King bed with her. If we put her on the pull-out in the living room, we can't use the kitchen or LR while she's sleeping, so the inflatable works better for us and she sleeps in it just fine.
Amanda