Remington does not make any for sure. They are a steel box with a plastic bottom so a person could try to construct a mag extension to replace the factory bottom.
I'm just gonna pull up a chair, crack open a beer and watch. It seems there might be a learning experience just around the corner ... Just as soon as somebody comes along and says "watch this" ...
I was originally thinking that I could take 2/3 factory mags and tig them together for the case. The concern is the spring, I would have to either make a new spring somehow or connect the springs together. I think they could connected using a pair of cable ferrules.
Yes, I am one of those guys that brainstorm how to make weird things happen.