Coach, here is something to think about IF you aren't building it on the playing field (inside your fence). We built a 4 station bullpen. Our pitcher throw in opposite directions so two are throwing one way and two are throwing 180 degrees opposite. We have portable backstops which our shop class built behind each. Note, these are straight backstops and not the type used for hitting. We placed our bullpen in an area where I could monitor both the bullpen and batting cages. In our situation, it is down the left field line. Why 4 throwing? Well, I can have 2 varsity pitchers throwing and 2 JV pitchers throwing AND coach all of them at the same time. I expect that when one pitcher is getting coached, they all are listening. Those 4 then do their other "work" like leg work together. The older, more seasoned pitchers, help train the lower level kids. This also allows me to have a point on the field where I can look while throwing bp and instruct pitching, hitting, etc. BTW, we have a volunteer coach that is always in the bullpen and so, no kid every throws without a coach present.