Take a 2x6 and make the floor box. Then take any left over pieces and make 4 wedges that you then can nail down once you get the barrel. It then can be adjusted for a bigger or smaller barrel. Not that the nice curved one wouldn't work, but I don't have a jigsaw and so what I can do with a circular saw is make squares and triangles.
And without being nearby to it right now, if you make a rack that is about two feet from end to end, you can fit a full 60 gallon barrel or a half 30 gallon barrel. If using my above patented wedges, you can then adjust for either size (in the future)