Product review (4 stars): *slightly* smaller diameter than mentioned, but does the job
Overview:
Con(?): Ever so slightly teenier dimensions than listed here (which benefited me)
Pro: Keeps things that should stay out of the disposal, out of the disposal - like rings, spoons, and cats.  Kitty's lack of opposable thumbs render him unable to out-muscle and out-smart the tool!

In order to keep our cat (sigh) from having his two front legs perpetually damp and covered with whoknowswhat kind of bacteria or microbes or invisible food bits (not to mention, keeping him away from the dangers of a disposal anyhow), I researched all the options for disposal stoppers.  Honestly, there was one different company's that I wanted more (better reviews, in my opinion), but after measuring our disposal opening and reading the measurements on my options, this is the only one that *might* fit.

Actually, according to the dimensions here, even it might not fit, but hoping that the rubber rings would be flexible enough, I figured it was worth the few dollars to give it a shot.

Well, this disposer tool is ever so slightly teenier in diameter than it mentions here, and in fact, it's slightly too tiny to be a snug fit for our sink.  The only downside of that is that it cannot ever work as a stopper, which is referenced here in the description and in some reviews.  It fits in perfectly, without slipping down past the rubber flanking in my own disposal, but even if I push it down "snug" - as far down as it will go - water still passes right on through.  I'm OK with that though - it is a better "problem" than what I was afraid would be the case.

So, I can keep it in the sink most of the time, including to run the disposal itself.  Yay!  Kitty is safe and my rings or forks or whatever will not fall into the depths of yikes.  I take it out when I am washing dishes to put a stopper in that actually works to hold water (a flat one that the cat has no problem removing, which is why I needed an item like this), or when I'm washing stuff down the drain.  Otherwise, food bits lay all over this, and that is just gnarly.  No one wants to have that.

I knocked a star off, not even so much for the size being ever so slightly off, but because I feel like surely another design would be even better.  The Stuffer, for instance, looks more right and less likely to get gross to me (I'm concerned with stuff growing around and in the rubber rings if not religiously washed and cared for), but, hey, even The Stuffer had some reviews indicating some design flaws.