Here's how I got the gray plastic ring out:
Tool:
I used a piece of 1/4" diameter AllThread, which is a 1/4" rod that is threaded along it's entire length (like a very long bolt).
It comes in various lengths but the shortest is probably three feet. (Most hardware stores carry it.) Of course it only has to be the length of the batteries plus another 6" to hold on to. Note that 5/16" all thread might have worked even better because it wouldn't flex at all.
Technique:
First let me describe the gray plastic ring that I extracted.
It could be described as two shapes molded into one part.
1. a flat disc with a hole in the center. 2. a very short tube (no more than 1/8" long).
Note that the hole in the center was not used to get ahold of the plastic ring at all.
The inside face of the short tube is what I grabbed, (using the threads on the tip of the AllThread).
I tipped the keyboard so that the battery compartment opening was down. I placed the AllThread up into the battery compartment so the the tip of the AllThread was resting on the inside face of the plastic ring's short tubular surface. I then used the AllThread to press the plastic tube against the side of the battery compartment so that the threads of the tip of the rod were pressing very firmly against the inside face of the plastic tube. (To Clarify: I applied lateral pressure only.) At that point I slowly pulled the AllThread (while maintaining sideways pressure) and the ring slowly came out.
Once It was out, I set the ring on top of the positive end of a battery with the flat disc part of the ring against the battery, so I could see down into the open tube of the plastic ring as it rested on the end of the battery. With the keyboard still positioned with the opening down, I slid the battery (with plastic ring on it) up into the compartment followed carefully by two more batteries (my keyboard takes three, though some only take two). Then I just put on the battery cap and the keyboard fired right up.
No problem.
I hope this helps.