The problem with a piece of plywood is that it typically doesn't stay standing by itself so I had to prop a chair beside the plywood, beside the staircase. You get the idea. A lot of crap at the bottom of the stairs and everytime I wanted to go up or down I had to hop this fence of assorted "stuff". It got irritating. Anyone who visited over the summer can tell you how hideous it looked.
I figured it was time to make something more permanent. It took a few pieces of 1"x6" and two hinges but I finally have a permanent "keep off my bed" gate!
I got the original idea from this website: http://www.remodelaholic.com/2012/09/diy-barn-door-baby-for-stairs/
The plans were for a baby gate but I used the general idea, scrapped the closing mechanism, and voila! When we installed it we tilted it slightly so it always swings closed. The cats have actually figured out how to push it open and run through before it closes on their tails.