I built this to keep our son from crawling up (and more importantly, from falling back down) the stairs. I wanted to avoid putting any holes in the wall or the newel post, and this is what I came up with. The box on the left end slips over the side of the newel, and has a dado for the gate to slide down into. The diagonal piece on the right just rests on the diagonal skirting and against the molding for the doorway. It too has a dado for the gate to slide down into.
We adults just stepped over it, and it worked pretty well for 5-6 months, until a little while after this picture was taken. It looks like he’s vaulting over it there, but I think he’s actually about to fall on his butt. It wasn’t long after that that he started climbing over it though, and I took it down.