Seadweller:
To get your enclosure resonably airtight you just need to put some form of the gasket between driver/enclosure and crossover panel/enclosure. The easiest way would be to buy window weatherstrip self adhesive tape (anything like this ) and use it as the gasket - it would probably take you less then an hour for both loudspeakers.
I also suggest you to try the crossover as it is posted (it was designed for GPA604-II in a 10cuft.) and then go for the adjustments, if necessary.