CurlyDave
Elite Member
Enough sand under the bottom of the p-trap to support the drain pipe against downward pressure if you should happen to push on it while putting the drain in.
Other than that, leave the box empty. You don't want to have to repair it, but if you have to you don't want to be chipping concrete out and possibly damaging the drain pipe.
Other than that, leave the box empty. You don't want to have to repair it, but if you have to you don't want to be chipping concrete out and possibly damaging the drain pipe.