I think a better approach is to begin with a larger volume than you need to fill your secondary. That means using a food grade bucket as your primary and aiming for a specific starting gravity rather than a specific volume. If you begin with say 1.125 - 1.25 gallons then you don't have to worry about how to fill the headroom when it comes time to rack (for some melomels you anticipate a great deal of sediment and so your starting volume is even larger). If you use a bucket as your primary then there is no problem of creating volcanos if you need to add nutrient, and assuming you simply loosely cover the top with a cloth or even the lid (loosely placed) there is no problem stirring daily either.