Well, Im not sure if its what you are looking for, but the one I carry hunting is made by butler creek (Belgrade, MI). called Speed Loader.
They are .50 cal. so the sabot fits tight and wont slide or rattle around inside.
They are hollow all the way through.
The thing I like about them is they are short, only about 2.5 inches. You could probably cut them shorter than that.
TCB