I saw last week-end that you can get the Galway Hooker in them 30L kegs. That's [i:re5waolk]only[/i:re5waolk] 50% bigger than a Corny, and may save you a lot of hassle.
In general, would it be wrong to fill a 30L keg with only 20L of beer ? If it's well topped up with CO2, that is...