I don't have the "stock" ballast, but I have used several sacks back there with no problems. I have a Skylon locker sack, it lays flatter, more like the original sack. It holds about 500 lbs, but I over inflate the crap out of it. I regularly inflate it to the point where it's pushing on the lid. If it popped, I'd put in a Skylon 750, no cover needed. My Locker sack cover ripped years ago, it's never leaked or had to be patched.