Yeah, it can be messy. Another way to do it is to stand it on end and fill the butt end with water - it absorbs down the staves easier that way, with the grain. After a couple of days of that (topping up the end as needed), flip it and do the other side for a couple of days. Then I might try filling it, after it's had a chance to swell for a while.
I haven't tried it this way, it's just an idea.
Maybe sit it in a kiddie pool and soak both ends at once?