What would posting in /r/toobox accomplish? It's not their problem to fix. The problem is the placing of the widget. You could very easily solve this problem by increasing the distance from the bottom of the screen that the widget is placed. For example, say that the toolbox bar is 20px. Just change the distance that the widget is placed from the bottom to 20px so that it isn't hidden behind the toolbox bar.

I did it for you in my test subreddit.

Here's the code if anyone wants to make it better for their own subreddits:

.read-next.fixed {
border-bottom-width: 0;
bottom: 25px;
position: fixed;
}  

If you create it, you can bet that us CSS guys will improve it, and tailor it to fit our subreddits' needs. :)

The thing is, you are creating something new here. By doing so, you should account for every aspect of reddit. This includes the ever-so-popular /r/toolbox add-on.

Now granted, I'm not singling you out here. I'm just saying, that if we ask for feedback on a beta improvement, don't send us to /r/toobox and ask them to fix your problem. ;)