if the barrel is too big around, make sure it comes with a lid. cut out a hole for the top you plan to use to sit over. or, cut it so that the top you plan to use sets down into the llid, and weld it in. if you just can't get the lid with it, i would put some bolts thru the sides at the level so that the top you want to use is at the height you want, and make some kind of filler to go around the top. maybe weld some metal to the top. sound doable to you?