If there isn't any cover on the flat....either way should work fine. The easiest thing to do will be to just take the buckets and fill them up with gravel, chunk rock, or anything else that you can find. I wouldn't spend any money on concrete unless you want to put vertical cover in it....
Take a 20' piece of rope and double it through the handle. Lower the bucket down and bounce it up and down with the rope to settle it on the bottom without knocking over the "stump" and spilling the rocks out. Then just release one side of the rope and pull the rope on out....then go to the next one.