Level one barrel, use a measuring cup (counting the amount you are adding) and fill on top till the water over flows. That will be the volume of material you need to mix up. Source a self setting material you might have. Lets say concrete, scuff the top area where you will me adding concrete, with 80grit sand paper ( you want to add "tooth" to surface for concrete to stick) fill with concrete or what ever material you come up with. Other option might be to try hot gluing a sheet plastic top on it.