What is a little strange is the picture on the course description shows the cinder block behind the barrel but the written description says in front of the barrel. I assume they will publish a clarification.
Yeah, it will probably vary by match. It would be nice to have a block on each side to keep it stable, but it might interfere with getting good rear support. If they put one behind the barrel I'd probably shoot of to the side of the barrel to make my rear bags fit.