This isn't exactly a roll off pit but our team designed and built a custom battery cart that makes things much easier for setting up the pit, and also makes it easier for eliminations for having the charged batteries closer to the field. We have the CAD file available online at our website
here.
It is able to hold 12 batteries, and can hold 4 of the 3-bank chargers AndyMark sells, however we have 2 of the 3-bank chargers and 3-single bank chargers for now. We usually have chargers for the drills and such set up on top of the cart when it is in the pits. We also have a battery spill kit that is always inside the cart.
Let me know if you have any questions about it.
-Nick