It’s a little hard to understand exactly what you are describing, but I would suggest some sort of block like you are thinking.
You can go to your local big box store and buy something called a cornice that would work, they are not cheap though. You can also try building a simple one.
Another option that shouldn’t be hard is to have the crown mold terminate itself buy just cutting it as if it were going around and outside corner when it’s really not.
You would have “short cut” on the left side of the long piece (just before the ceiling goes up). Then you would cut another small piece, probably 2”-3”, that is square on the left side and has a “short cut” on the right side. THis would just make a small corner and make the moulding look like it could be going back through the wall.
Just found a photo of it! Check this out.
http://extremehowto.com/wp-content/uploads/2012/04/To-terminate-a-run-of-moulding.jpg